Check that target supports atomic pointer operations before defining max size for Arc

[sjudson]

Resolving #167 cc: @jamesmunns

When building with, e.g., --target riscv32i-unknown-none-elf and -F alloc on the current main branch, the build will fail with

error[E0432]: unresolved import `alloc::sync`
 --> source/postcard/src/max_size.rs:5:33
  |
5 | use alloc::{boxed::Box, rc::Rc, sync::Arc};
  |                                 ^^^^ could not find `sync` in `alloc`

due to the lack of support on that target for atomic pointer operations (see: https://doc.rust-lang.org/alloc/sync/index.html).

Following the recommendation from the Rust docs, this gates definition of POSTCARD_MAX_SIZE for alloc::sync::Arc behind a configuration check to confirm it is available on the target first.

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[jamesmunns]

Gotcha! And you can confirm this resolves the issue you saw in #167? If so, I'll merge and release after CI is green.

[jamesmunns]

Also @sjudson if you'd like to add a test for this arch (rv32 + alloc) to the CI so it doesn't regress in the future: https://github.com/jamesmunns/postcard/blob/main/ci.sh feel free to add it to this PR!

[jamesmunns]

I went ahead and added it :)

[sjudson]

Thanks!