[PATCH] Cygwin: Fix compatibility with GCC 15

[Thirumalai Nagalingam]

Hello,

Please find my patch attached for review.

Summary of Changes:

- GCC 15 defaults to `-std=gnu23`, causing build failures in `testsuite`
  due to outdated C function declarations. This patch updates the function
  declarations to align with modern standards.
- Introduced `cpu_relax.h` to support `cancel3` and `cancel5` tests by
  providing architecture-specific instructions.

Thanks & regards
Thirumalai Nagalingam
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