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

[Raul Tambre via cfe-commits]

tambry wrote:

@MaskRay `-masm=intel` switches the `asm()` blocks to Intel syntax too. That breaks a lot more code and results in a rather unworkable amount of LLVM test failures. My main interest has been in getting assembly output from Clang for debugging performance issues so `-mllvm -x86-asm-syntax=intel` is more suitable.

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