RISC-V: Change flags for stable extensions to non-experimental

Flags for extensions with 'kernel' support is auto-enabled.
We consider these extensions stable (all may not yet have been ratified, but any moment).

Ratified and/or stable (exists in kernel and compiler):
UseRVA22U64 UseRVV, UseZba, UseZbb, UseZbs, UseZic64b, UseZicbom, UseZicbop
UseZicboz, UseZihintpause, UseZtso, UseZvkb, UseZvknha, UseZvknhb

And have bake-time:
UseRVV, UseZba, UseZbb and UseZbs