[NFCI][lldb][test][asm] Enable AT&T syntax explicitly (#172358)
Implementation files using the Intel syntax typically explicitly specify it.
Do the same for the few files using AT&T syntax.
This enables building LLVM with `-mllvm -x86-asm-syntax=intel` in one's Clang config files
(i.e. a global preference for Intel syntax).
Unreverts: 14c69497b31038b37c273417f43bd2cfe169c86f