Work around removal of some PowerPC intrinsics in GCC 15

[musicinmybrain]

Fixes #611.

Checklist

• I have read the contributing guidelines.
• I have considered portability of my change across platforms and architectures.
• I have self-reviewed my code.
• I have commented my code where necessary.
• I have updated the documentation accordingly. N/A
• I have added tests that prove my fix is effective or that my feature works. N/A

What is the purpose of this pull request?

• Fix a bug

What changes did you make?

On GCC 15 and later, fall back to inline assembly instead of using removed intrinsics __builtin_vsx_xvcvsxwdp() and __builtin_vsx_xvcvdpsxws().

This PR works around the removal of PowerPC intrinsics __builtin_vsx_xvcvsxwdp() and __builtin_vsx_xvcvdpsxws() in GCC 15.

Does this PR relate to any existing issue?

Relates to/Fixes #611

Is there anything you would like reviewers to focus on?

Consider whether it would be better to replace __builtin_vsx_xvcvsxwdp() with vec_doublee/vec_doubleo as suggested in gcc-mirror/gcc@fd9fdb3, and to replace __builtin_vsx_xvcvdpsxws() with vec_signede/vec_signedo as suggested in gcc-mirror/gcc@224cc56. As noted in #611, I’m not prepared to contribute that approach, but I would be happy to help test it.