[RISCV] Reject 'g' with explicit version in parseArchString

There is no versioning scheme for the 'g' shorthand for imafd (or in
current ISA specs, imafd_zifencei_zicsr). As such, the only sensible
behaviour to me seems to be to reject a version for it.

Differential Revision: https://reviews.llvm.org/D145954