This fixes failures in AArch64 buildbots by running the clang/test/CodeGen/X86/att-inline-asm-prefix.c only when the X86 target is available.
For now, we lost the encoding information if we using inline assembly. The encoding for the inline assembly will keep default even if we add the vex/evex prefix. Differential Revision: https://reviews.llvm.org/D90009