test/chacha: added ELFv2 ABI FPR preservation check for POWER10 8x path

test/chacha: added ELFv2 ABI FPR preservation check for POWER10 8x path

On POWER10, ChaCha20_ctr32_vsx_8x is activated for buffers over 255
bytes and uses vxxlor to alias FPR14-FPR25 as temporary storage. Add a
test to chacha_internal_test that pins known values in f14-f25 via
inline asm, calls through ChaCha20_ctr32 with a 512-byte buffer to
trigger the 8x path, and verifies the registers still hold their
original values. The test is gated on PPC_BRD31 (POWER10 capability
flag) so it is skipped silently on older hardware.

Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
MergeDate: Sat Apr 11 20:13:00 2026
(Merged from https://github.com/openssl/openssl/pull/30587)