[RISCV] Add CSRs defined in the recently ratified Sstc extension

The 'RISC-V "stimecmp / vstimecmp" Extension' was ratified at the end of
last year though hasn't yet been integrated in the main specification
documents (see
<https://wiki.riscv.org/display/TECH/Recently+Ratified+Extensions>).

RISC-V "stimecmp / vstimecmp" Extension
<https://github.com/riscv/riscv-time-compare/releases/download/v0.5.4/Sstc.pdf>.

Differential Revision: https://reviews.llvm.org/D117311