[RISCV] Add mcountinhibit CSR

Summary:
The mcountinhibit CSR is defined in the ratified 1.11 version of the privileged
spec.

Reviewers: apazos, asb, lenary, luismarques

Reviewed By: asb

Subscribers: hiraditya, rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, s.egerton, sameer.abuasal, evandro, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82913