[llvm] [RISCV] Add Enum for CSR encodings. (PR #121674)

[Sudharsan Veeravalli via llvm-commits]

svs-quic wrote:

hi Craig, we are running into compilation failures in our downstream compiler due to this change. We have custom CSRs that have a `qc.` prefix and this is causing tablegen to error out while creating the `GenSearcherTables.inc` file.

The error looks something like:

```
build/riscv-build/lib/Target/RISCV/RISCVGenSearchableTables.inc:458:5: error: expected '= constant-expression' or end of enumerator definition
  qc.<customcsr> = 1984
      ^
```

I have pushed #125172 which fixes this.

https://github.com/llvm/llvm-project/pull/121674