[llvm] [RISCV] Gate CSR defined by experimental and vendor extensions (PR #104424)

[Jesse Huang via llvm-commits]

jaidTw wrote:

I decided to make this patch updates the error message only to let it proceed, and we may split the unratified and vendor CSRs until we really have them in the upstream.

https://github.com/llvm/llvm-project/pull/104424