Incorrect pthread_t definition on Darwin (should be a raw pointer, not an uintptr_t)

[highjeans]

Description

This PR Fixes #2903, however I am not sure if the change to build.rs is the right way to do it. MacOS has the _opaque_pthread_t type, however in build.rs it gets classified as a type rather than a struct, resulting in the test case for it using _opaque_pthread_t rather than struct _opaque_pthread_t. Please let me know if there is a better way to test _opaque_pthread_t without having to modify the build.rs file.

Sources

https://github.com/apple-oss-distributions/xnu/blob/8d741a5de7ff4191bf97d57b9f54c2f6d4a15585/EXTERNAL_HEADERS/sys/_pthread/_pthread_types.h#L103
https://github.com/apple-oss-distributions/libpthread/blob/main/include/sys/_pthread/_pthread_t.h#L31

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

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