[RISCV] Add extension information to RISCVFeatures.td. NFC

This adds a new RISCVExtension class that inherits from SubtargetFeature.
This contains the major/minor version and whether the extension is
experimental. The plan is to use this to generate the tables for
RISCVISAInfo.cpp.

The version numbers might not be accurate yet. If there are errors
they will be fixed before they are used for anything. It will be easier
to verify once the new tablegen backend is written to generate the
RISCVISAInfo.cpp table.

Author: topperc