emit `.att_syntax` when global/naked asm use that option
fixes https://github.com/rust-lang/rust/issues/143542
LLVM would error when using `-Cllvm-args=-x86-asm-syntax=intel` in combination with global/naked assembly with `att_syntax`. It turns out that for LLVM you do in this case need to emit `.att_syntax`.
r? `@Amanieu`