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

[Mark Zhuang via llvm-commits]

zqb-all wrote:

> I did something similar in #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.

Thanks!  I found that my implementation would not preferentially use the CSR name when the `.set` and CSR name conflicted.
So eventually I used your implementation method in the https://github.com/llvm/llvm-project/pull/67377.

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