[RISCV] Add Smcsrind and Sscsrind extension (#93952)

Specification link:
https://github.com/riscv/riscv-isa-manual/blob/main/src/indirect-csr.adoc


Some CSRs (`*ireg` and `*iselect`) in Smcsrind/Sscsrind extensions are
originally defined as part of the Smaia/Ssaia extensions and are already
supported in LLVM. The missing CSRs (`*ireg2` to `*ireg6` for `m`, `s`,
and `vs`) are added in this PR.