[RISCV] Add -march support for many of the S extensions mentioned in the...

[RISCV] Add -march support for many of the S extensions mentioned in the profile specification. (#79399)

This is a good portion of the extensions mentioned in the RVA23 profile
here
https://github.com/riscv/riscv-profiles/blob/main/rva23-profile.adoc

I don't believe these add any new CSRs. Sstc does add new CSRs, but we
already added them without the extension name a while back.

I tried to keep the descriptions in RISCVFeatures.td fairly short since
the strings show up in `-print-supported-extensions`.