[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