[AArch64] Allow PAC mnemonics in the HINT space with PAC disabled

Summary:
It is important to emit HINT instructions instead of PAC ones when
PAC is disabled. This allows compatibility with other assemblers
(e.g. GAS). This was implemented in commit da33762d.

Still, developers of assembly code will want to write code that is
compatible with both pre- and post-PAC CPUs. They could use HINT
mnemonics, but the new mnemonics are a lot more readable (e.g.
paciaz instead of hint #24), and they will result in the same
encodings. So, while LLVM should not *emit* the new mnemonics when
PAC is disabled, this patch will at least make LLVM *accept*
assembly code that uses them.

Reviewers: danielkiss, chill, olista01, LukeCheeseman, simon_tatham

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78372