[LLDB][RISCV] Add RVV registers enums

[LLDB][RISCV] Add RVV registers enums

RVV stands for "RISC-V V Extension", which adds 32 vector registers, and seven unprivileged CSRs (vstart, vxsat, vxrm, vcsr, vtype, vl, vlenb) to a base scalar RISC-V ISA.

The base vector extension is intended to provide general support for data-parallel execution within the 32-bit instruction encoding space, with later vector extensions supporting richer functionality for certain domains.

Reviewed By: DavidSpickett, kito-cheng

Differential Revision: https://reviews.llvm.org/D141898