[RISCV] Enable the use of the old mucounteren name
The RISC-V Privileged Specification 1.11 defines mcountinhibit, which
has the same numeric CSR value as mucounteren from 1.09.1. This patch
enables the use of the old mucounteren name.
Patch by Yuichi Sugiyama.
Reviewed By: lenary, jrtc27, pzheng
Differential Revision: https://reviews.llvm.org/D85067