[libcxxabi] [libunwind] [NFCI][test][asm] Enable AT&T syntax explicitly (PR #166818)

[Nico Weber via cfe-commits]

nico wrote:

> 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).

FYI: Just `-masm=intel` should work too, no need for relying on an internal mllvm flag.

https://github.com/llvm/llvm-project/pull/166818