[llvm] [RISCV][MC] Implement ISA mapping symbols (PR #67541)

[Andreu Carminati via llvm-commits]

andcarminati wrote:

I think this PR needs to be updated. For some reason toFeatureVector is missing in RISCVISAInfo, maybe some diff/appy issue.

https://github.com/llvm/llvm-project/pull/67541