[llvm] [NFCI][bolt][test] Enable AT&T syntax explicitly (PR #167225)

[Raul Tambre via llvm-commits]

tambry wrote:

@maksfb A vast majority of the tests don't use the Clang frontend to compile assembly so they're not affected. Across the whole LLVM codebase only a few tens of changes have been necessary (~30'ish).

I've switched to passing `-mllvm -x86-asm-syntax=att` explicitly in the lit local config and removed the explicit annotations per your preference.  
Worth noting that #167234 would add these to all LLVM MC generated outputs going forward for better interoperability, and not just only for Intel. Explicit annotations would track somewhat better with that and is what the fixes elsewhere went with. To me that also seems cleaner than passing a x86-specific flag on all architectures, ignoring some accidental breakages while the explicit syntax output change hopefully gets merged and propagates.

https://github.com/llvm/llvm-project/pull/167225