[libunwind][PPC64] Fix unw_getcontext corrupting callee-saved VSX registers on LE (#198371)

This is the first of two independent fixes for libunwind on ppc64le
(ELFv2 ABI, little-endian), where two separate bugs together cause
SIGSEGV during backtracing. This commit addresses the VSX register
corruption; the TOC-restore fault is handled in a follow-up. Both
were discovered while debugging lang/rust build failures with
RUST_BACKTRACE=1 on FreeBSD/powerpc64le (IBM POWER9).

On ppc64le, `unw_getcontext` saves each VS register with an in-place
`xxswapd n, n` followed by `stxvd2x`. The swap is needed because
`stxvd2x` stores doublewords in the wrong order on LE. However, the
macro never applies a second `xxswapd` to restore the register after
the store, so all 64 VS registers are permanently corrupted on return
from `unw_getcontext`.

This affects every callee-saved VSX register: f14-f31 (VSR14-VSR31)
and VR20-VR31 (VSR52-VSR63). After `_Unwind_Backtrace` returns, any
code that uses these registers sees wrong values. In practice this
manifests as SIGSEGV inside hashbrown's `reserve_rehash`: VR20-VR31
are corrupted before a SIMD comparison loop runs, producing an
out-of-bounds access.

Fix: add a second `xxswapd n, n` after the `stxvd2x` store. Since
`xxswapd` is its own inverse, the pair is a no-op on the architectural
register while still writing the correctly byte-swapped value to memory.
GitOrigin-RevId: 6b0a46958c56d91e3b2fd5ffecfe163dde1fac3a