[llvm] X86: Support dollar symbol for AT&T syntax and add output-asm-variant to llc (PR #147876)

[LIU Hao via llvm-commits]

lhmouse wrote:

In your commit I suspect this will not be correct: `calll ($hen at PLT)`
It should be `calll ($hen)@PLT`. The modifier is not to be mistaken as part of the symbol.

As said earlier the at&t syntax is just nonsense like in `movl	($a)+16(%rip), %eax`. Only the symbol itself should be quoted.


https://github.com/llvm/llvm-project/pull/147876