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

[Maksim Panchenko via llvm-commits]

maksfb wrote:

> FYI, enabling X86 targeting on AArch64 just leads to another test case failing:

Thanks - I'll take a look. Can you try #169541 meanwhile?

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