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

The "RISC-V State Enable Extension" was ratified at the end of 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>).

This commit adds the CSRs defined by this extension in
<https://github.com/riscv/riscv-state-enable/releases/download/v0.6.3/Smstateen.pdf>.

Differential Revision: https://reviews.llvm.org/D117310