[LLDB][RISCV] Add RVM and RVA instruction support for EmulateInstructionRISCV

Add:
- RVM and RVA instructions sets.
- corresponding unittests.

Further work:
- implement RVC, RVF, RVD, and RVV extension.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D133670