[RISCV] Accept zicntr and zihpm command line options

This change adds the definition of the two extensions, but does not either a) make any register definitions conditional on them or b) enabled the extensions by default.

This is somewhat analogous to https://reviews.llvm.org/D143953, but with some key differences.  The best discussion I can find on status is here: https://github.com/riscv/riscv-profiles/issues/43.  These were removed between document version 2.1 and 2.2, but were not defined as new extensions in 2.2.  That addition came later - in March 2022.

According to https://drive.google.com/file/d/1qa57pePesOiDOrNzxuuGFhCL4Rbi9AYB/view these were ratified in March 2023.

Reviewed By: asb, reames

Differential Revision: https://reviews.llvm.org/D144215