[llvm] [RISCV][MC] Support imm symbol in parseCSRSystemRegister (PR #112007)

[Alexander Richardson via llvm-commits]

https://github.com/arichardson commented:

This looks good to me. Since this uses some of my code could you please add `Co-authored-by: Alex Richardson <alexrichardson at google.com>` to the commit message.

https://github.com/llvm/llvm-project/pull/112007