[LLDB][RISCV] Add RVDC instruction support for EmulateInstructionRISCV

[LLDB][RISCV] Add RVDC instruction support for EmulateInstructionRISCV

RVC is the RISC-V standard compressed instruction-set extension, named "C", which reduces static and dynamic code size by adding short 16-bit instruction encodings for common operations, and RVCD is the compressed "D extension".

And "D extension" is a double-precision floating-point instruction-set extension, which adds double-precision floating-point computational instructions compliant with the IEEE 754-2008 arithmetic standard.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D140961