[RISCV] Implement new architecture extension macros

This adds support for the new architecture extension test macros as
defined in the C-API Document:
https://github.com/riscv/riscv-c-api-doc/blob/master/riscv-c-api.md

Extension versions have been taken from what are used in
RISCVTargetStreamer for ratified extensions, and the -march parser
for experimental extensions.

Differential Revision: https://reviews.llvm.org/D94403