[RISCV] Update error message to not call 'RV32' and 'RV64' an extension.

I used RV32 so I didn't have to write RV32I and RV32E. Ideally
these builtins will be wrapped in a header someday so long term I don't
expect users to see these errors.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D133444