[NFCI][asm][i386/x86-64] Enable AT&T syntax explicitly (#164453)
Implementation files using the Intel syntax explicitly specify it.
Do the same for the few files using AT&T syntax.
This also enables building LLVM with `-mllvm -x86-asm-syntax=intel` in one's Clang config files
(i.e. a global preference for Intel syntax).
No functional change intended.