add an assembler to the toolchain

[andrewrk]

Prerequisite for #16270.

Builds of Zig that do not link against LLVM and Clang still need to be able to compile assembly files.

The existing commands already work, and they already support compiling assembly files: zig build-obj, zig build-exe, zig build-lib. The logic needs to be modified to use Zig's own assembler rather than invoking Clang as a subprocess.

For the x86 family specifically, let us jump on the intel syntax train, embracing that as the better syntax. However, we also want to be able to compile the multitude of existing files from the wild without any changes. So it will need to support AT&T syntax as well.

I suggest we start by borrowing LLVM's CPU instruction data via another tool in the tools/ directory. At some point the backends should start using this data as well instead of using an ad-hoc parser, but that will be a follow-up issue.

In order to close this issue, Zig must use its own assembler for all input files, never calling the clang binary for assembly.

Related:

• parse inline assembly syntax according to a set of dialects; integrate inline assembly more closely with the zig language #10761
• CPU features are not passed to clang when assembling #10411