This PR makes a number of changes to the UNIX randomness implementation:
* Use `io::Error` for centralized error handling
* Move the file-fallback logic out of the `getrandom`-specific module
* Stop redefining the syscalls on macOS and DragonFly, they have appeared in `libc`
* Add a `OnceLock` to cache the random device file descriptor

to pick up changes to `./x miri` and `./x test` commands

Moves the CFI ui tests to the cfi directory and removes the cfi prefix
from tests file names similarly to how the cfi codegen tests are
organized.

…gets

Refactor `powerpc64` call ABI handling

As the [specification](https://openpowerfoundation.org/specifications/64bitelfabi/) for the ELFv2 ABI states that returned aggregates are returned like arguments as long as they are at most two doublewords, I've merged the `classify_arg` and `classify_ret` functions to reduce code duplication. The only functional change is to fix rust-lang#128579: the `classify_ret` function was incorrectly handling aggregates where `bits > 64 && bits < 128`. I've used the aggregate handling implementation from `classify_arg` which doesn't have this issue.

`@awilfox` could you test this on `powerpc64-unknown-linux-musl`? I'm only able to cross-test on `powerpc64-unknown-linux-gnu` and `powerpc64le-unknown-linux-gnu` locally at the moment, and as a tier 3 target `powerpc64-unknown-linux-musl` has zero CI coverage.

Fixes: rust-lang#128579

…Denton

std: refactor UNIX random data generation

This PR makes a number of changes to the UNIX randomness implementation:
* Use `io::Error` for centralized error handling
* Move the file-fallback logic out of the `getrandom`-specific module
* Stop redefining the syscalls on macOS and DragonFly, they have appeared in `libc`
* Add a `OnceLock` to cache the random device file descriptor

…ubilee

Remove unused lifetime parameter from spawn_unchecked

Amanieu caught this when reviewing the stabilization proposal in rust-lang#55132.

The `'a` lifetime here is useless. The signature is asking the caller of `spawn_unchecked` to "give me any lifetime that is shorter than your F's and T's lifetime", which they can always to with no effect, because arbitrarily short lifetimes exist.

bootstrap: don't use rustflags for `--rustc-args`

r? `@onur-ozkan`

This is going to require a bit of context.

rust-lang#47558 has added `--rustc-args` to `./x test` to allow passing flags when building `compiletest` tests. It was made specifically because using `RUSTFLAGS` would rebuild the compiler/stdlib, which would in turn require the flag you want to build tests with to successfully bootstrap.

rust-lang#113178 made the request that it also works for other tests and doctests. This is not trivial to support across the board for `library`/`compiler` unit-tests/doctests and across stages. This issue was closed in rust-lang#113948 by using `RUSTFLAGS`, seemingly incorrectly since rust-lang#123489 fixed that part to make it work.

Unfortunately rust-lang#123489/rust-lang#113948 have regressed the goals of `--rustc-args`:
- now we can't use rustc args that don't bootstrap, to run the UI tests: we can't test incomplete features. The new trait solver doesn't bootstrap, in-progress borrowck/polonius changes don't bootstrap, some other features are similarly incomplete, etc.
- using the flag now rebuilds everything from scratch: stage0 stdlib, stage1 compiler, stage1 stdlib. You don't need to re-do all this to compile UI tests, you only need the latter to run stdlib tests with a new flag, etc. This happens for contributors, but also on CI today. (Not to mention that in doing that it will rebuild things with flags that are not meant to be used, e.g. stdlib cfgs that don't exist in the compiler; or you could also imagine that this silently enables flags that were not meant to be enabled in this way).

Since then, rust-lang@bd71c71 has started using it to test a stdlib feature, relying on the fact that it now rebuilds everything. So rust-lang#125011 also regressed CI times more than necessary because it rebuilds everything instead of just stage 1 stdlib.

It's not easy for me to know how to properly fix rust-lang#113178 in bootstrap, but rust-lang#113948/rust-lang#123489 are not it since they regress the initial intent. I'd think bootstrap would have to know from the list of test targets that are passed that the `library` or `compiler` paths that are passed could require rebuilding these crates with different rustflags, probably also depending on stages. Therefore I would not be able to fix it, and will just try in this PR to unregress the situation to unblock the initial use-case.

It seems miri now also uses `./x miri --rustc-args` in this incorrect meaning to rebuild the `library` paths they support to run with the new args. I've not made any bootstrap changes related to `./x miri` in this PR, so `--rustc-args` wouldn't work there anymore. I'd assume this would need to use rustflags again but I don't know how to make that work properly in bootstrap, hence opening as draft, so you can tell me how to do that. I assume we don't want to break their use-case again now that it exists, even though there are ways to use `./x test` to do exactly that.

`RUSTFLAGS_NOT_BOOTSTRAP=flag ./x test library/std` is a way to run unit tests with a new flag without rebuilding everything, while with rust-lang#123489 there is no way anymore to run tests with a flag that doesn't bootstrap.

---
edit: after review, this PR:
- renames `./x test --rustc-args` to `./x test --compiletest-rustc-args` as it only applies there, and cannot use rustflags for this purpose.
- fixes the regression that using these args rebuilt everything from scratch
- speeds up some CI jobs via the above point
- removes `./x miri --rustc-args` as only library tests are supported, needs to rebuild libstd, and `./x miri --compiletest-rustc-args` wouldn't work since compiletests are not supported.

Slightly refactor `TargetSelection` in bootstrap

Mostly a drive-by refactoring of `TargetSelection` to reduce some manual "windows-gnu" detection and also accesses to the `triple` field.

r? `@onur-ozkan`

…-to-cfi-directory, r=compiler-errors

CFI: Move CFI ui tests to cfi directory

Move the CFI ui tests to the cfi directory and removes the cfi prefix from tests file names similarly to how the cfi codegen tests are organized.

Fix blessing of rmake tests

Fixes rust-lang#129038.

When running in `--bless` mode, we now set the value of `RUSTC_BLESS_TEST` to the current test's source directory. This allows the diff helper in `run_make_support` to find the original snapshot file in the source directory and bless that, instead of unhelpfully blessing the temporary copy in `build`.

r? `@jieyouxu`