x86-64 Intel syntax support for --emit=asm

[HertzDevil]

There should be a way to enable Intel syntax for the assembly file emitted by LLVM when building an x86-64 program, regardless of whether asm accepts AT&T or Intel syntax and, when cross-compiling, probably regardless of whether the host platform runs on x86-64 too.

The main use case, of course, is getting the relevant option in Compiler Explorer to work, but it also might be useful to feed the assembly file into assemblers that only support the Intel syntax, like MASM.

Note that asm itself cannot use Intel syntax until #14264.