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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

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

-mattr is the LLVM internal notion of "attributes", not the architecture extension names defined by Arm.  They mostly match, but not completely, for historical reasons.

> So, can that just be changed?

It's fine if the diagnostics don't match the -mattr flag.

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