[llvm] [clang] [RISCV][MC] MC layer support for the experimental zalasr extension (PR #79911)

[Philip Reames via llvm-commits]

preames wrote:

> It has been approved to be pursued as a fast track extension (https://lists.riscv.org/g/tech-unprivileged/topic/arc_architecture_review/101951698). It has not yet been approved by the chairs.

Works for me.  I've edited the PR description to include this information so that it ends up in the submit message.

I'm about to land this change on @mehnadnerd behalf since he doesn't have commit access.  This was requested out of band.  

https://github.com/llvm/llvm-project/pull/79911