gh-130115: fix thread identifiers for 32-bit musl

[vfazio]

CPython's pthread-based thread identifier relies on pthread_t being able to be represented as an unsigned integer type.

This is true in most Linux libc implementations where it's defined as an unsigned long, however musl typedefs it as a struct *.

If the pointer has the high bit set and is cast to PyThread_ident_t, the resultant value can be sign-extended (https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/Arrays-and-pointers-implementation.html). This can cause issues when comparing against threading._MainThread's identifier. The main thread's identifier value is retrieved via _get_main_thread_ident which is backed by an unsigned long which truncates sign extended bits.

  >>> hex(threading.main_thread().ident)
  '0xb6f33f3c'
  >>> hex(threading.current_thread().ident)
  '0xffffffffb6f33f3c'

Work around this by making a new function which translates a pthread_t to PyThread_ident_t by casting through an integer type of the appropriate size to avoid sign extension. Factoring this out allows us to replace the internal logic in the future with some min-heap or other data structure to manage pthread_t objects in a more opaque fashion.

Note musl isn't "officially" supported in PEP 11, however platform detection was added in c163d7f and similar PRs have been merged in the past which target it 5633c4f

This PR is intended to be a "minimum" to get this working. Longer term there should maybe be work to keep pthread_t opaque and not make assumptions about its type.

---

• Issue: Active thread list may be inaccurate due to data type mismatch #130115