[RISCV] Cleanups in CORE-V (xcv) extensions

This is a mostly NFC change cleaning up and clarifying components of the
in-tree CORE-V (xcv*) extensions following discussions on the remaining
extensions.

This makes the following changes to the xcbitmanip and xcvmac support:

1. Add missing extensions from RISCVISAInfo, such that they can be
   supported in clang's -march option.
2. Clarify the extension version number is 1.0.0 in documentation.
3. Clarify the extensions are by OpenHW Group, and the capitilization
   of the CORE-V extension family.
4. Add CORE-V to extension name in RISCVFeatures, both to be consistent
   with other vendors, and also better distinguish e.g. CORE-V bit
   manipulation vs RISC-V's standard Zb extensions.

Differential Revision: https://reviews.llvm.org/D155283