Auto merge of #84677 - CDirkx:pal, r=Mark-Simulacrum

Fix `tidy` platform-specific code check

I noticed new platform-specific code was introduced outside of `std::sys` ([example](https://github.com/rust-lang/rust/blob/master/library/std/src/thread/available_concurrency.rs)), which should have been checked against by `tidy`. Apparently there are 2 problems with the current check implementation:

- It ignores everything after encountering "mod tests", which is often at the very top of a file.
- There was a bug where when checking the byte immediately before a found string, the first byte of the file was checked instead.

I fixed the bug and made excluding tests a bit more robust by instead adding the following rules:
- Files with a path containing either `tests` or `benches` are excluded.
- A `cfg(...)` containing `test` is excluded.

(Tests are excluded because almost all tests have something like `#[cfg(not(target_os = "emscripten"))]` somewhere.)

The fixed check found some more cases of platform-specific code; for now I have explicitly excluded them and added a FIXME stating that the platform-specific code must be moved to `sys`.

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