[llvm] [AArch64] support `.arch_extension dit` (PR #169999)

[David Green via llvm-commits]

davemgreen wrote:

GCC doesn't support this (https://gcc.godbolt.org/z/EcvKh4Tjv), but if there is a -march option then it sounds OK to add one. The other alternative is to remove the need for +dit, like GCC has it. See for example #163166. I believe there is the idea that sys-reg only extensions do not give an error, as you can only write the assembly where you know they are useful. I think either is fine, I'm not sure what others think and whether I've mis-remembered the idea of MSR sysregs.

https://github.com/llvm/llvm-project/pull/169999