[llvm] [RISCV][MC] MC layer support for the experimental zalasr extension (PR #69685)

[Alex Bradbury via llvm-commits]

asb wrote:

Zalasr is using the namespace reserved for standard extensions, but doesn't seem to be being developed under github.com/riscv - why is that? Is the repo going to get moved to that org? I see it _is_ an official in-development extension based on your riscv-opcodes PR, but it would be great to remove that ambiguity.

https://github.com/llvm/llvm-project/pull/69685