[compiler-rt] [libunwind] [openmp] [NFCI][asm][i386/x86-64] Enable AT&T syntax explicitly (PR #164453)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay approved this pull request.

libunwind changes look good. Please wait for approval from a sanitizer maintainer

You can use `clang -masm=intel` instead of `-mllvm -x86-asm-syntax=intel`

https://github.com/llvm/llvm-project/pull/164453