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

[Alexander Richardson via llvm-commits]

https://github.com/arichardson commented:

I did something similar in https://github.com/llvm/llvm-project/pull/67377 but forgot about this change so didn't update it. It might be a good idea to see if you can reuse any of the tests there.

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