[RISCV] Add MC layer support for proposed Bit Manipulation extension (version 0.92)

This adds the instruction encoding and mnenomics for the proposed
RISC-V Bit Manipulation extension (version 0.92). It is implemented with
each category of instruction as its own target feature, with the 'b'
extension feature enabling all options. Since this extension is not yet
ratified, all target features are prefixed with 'experimental-' to note
their status.

Differential Revision: https://reviews.llvm.org/D65649