[llvm] [RISC-V][MC] Accept an absolute variable value as a CSR number (PR #67377)

[Alexander Richardson via llvm-commits]

arichardson wrote:

The RISC-V test suite changes were merged, so it would be good to also get the LLVM side side changes landed (and maybe even backported to 17?)

This is a rather small change so hopefully that's ok.

https://github.com/llvm/llvm-project/pull/67377