[libc++abi][libunwind] Enable AT&T syntax explicitly (#166818)

Implementation files using the Intel syntax typically explicitly specify
it. Do the same for the few files where applicable for AT&T.

This enables building LLVM with `-mllvm -x86-asm-syntax=intel` in one's
Clang config files (i.e. a global preference for Intel syntax).

NOKEYCHECK=True
GitOrigin-RevId: afc83688cfadfb07c1cd9edc4f3c855d7cf4a261