[LLDB][RISCV] Add DWARF Registers

According to [RISC-V DWARF Specification](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-dwarf.adoc) add RISCV DWARF Registers.

Don't worry about the difference between riscv32 and riscv64, they just have different bytes of registers.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D130686