[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.

GitOrigin-RevId: c208a23643231d0b19c6f795895a16dfe6797340