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

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

@tambry @maksfb @paschalis-mpeis - this appears to break tests unless the x86 backend is enabled as a part of LLVM_TARGETS_TO_BUILD. Can we entertain a revert or is there another way forward?

The argument is defined in an x86 target module:

https://github.com/llvm/llvm-project/blob/1c3b10f2e2d8f9600fedd5e579aef69d7d31fadc/llvm/lib/Target/X86/MCTargetDesc/X86MCAsmInfo.cpp#L26-L30

(e.g. tests now fail if -DLLVM_TARGETS_TO_BUILD=AArch64 is supplied to cmake):

```
******************* TEST 'BOLT :: timers.c' FAILED ********************
...
# .---command stderr------------
# | clang (LLVM option parsing): Unknown command line argument '-x86-asm-syntax=att'.  Try: 'clang (LLVM option parsing) --help'
# | clang (LLVM option parsing): Did you mean '--asan-stack=att'?
# `-----------------------------
# error: command failed with exit status: 1
```



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