[LLDB][RISCV] Make software single stepping work

Add:
- `EmulateInstructionRISCV`, which can be used for riscv32 and riscv64.
- Add unittests for EmulateInstructionRISCV.

Note: Compressed instructions set (RVC) was still not supported in this patch.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D131759