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

[Folkert de Vries via llvm-commits]

folkertdev wrote:

Looks like the `complxnum` name is already visible to users also as a command line argument

https://github.com/llvm/llvm-project/blob/main/llvm/test/MC/AArch64/armv8.3a-complex_missing.s

So, can that just be changed?

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