[PATCH] Cygwin: Fix compatibility with GCC 15

[Jon Turney]

On 27/03/2025 12:41, Thirumalai Nagalingam wrote:
> 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    due to outdated C function declarations.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.

Applied, many thanks!

I tweaked the whitespace a bit and split the fix ARM64 processor idling 
during the cancellation tests into a separate patch.