[llvm] [llvm][mc][riscv] MC support of T-Head vector extension (xtheadvector) (PR #84447)

[via llvm-commits]

kata-ark wrote:

> > Last week we opened a [discussion on discourse](https://discourse.llvm.org/t/questions-about-the-t-head-vector-extension-xtheadvector-in-llvm-upstream/77298/2), and this PR may answer some questions there. Any comments and suggestions are welcomed and appreciated!!
> 
> It does help with that, thank you. Given we started discussion there it would probably be best to try to keep discussion around the policy question of taking in patches supporting this extension in that thread, so discussion doesn't get spread out all over the place. I've fed back a question that came up in the last RISC-V LLVM sync-up call there.

Okay, let's move the discussion to where it started. 😉 

https://github.com/llvm/llvm-project/pull/84447