[LLDB][RISCV] Add RISC-V ArchSpec and rv32/rv64 variant detection

Adds the RISC-V ArchSpec bits contributed by @simoncook as part of D62732,
plus logic to distinguish between riscv32 and riscv64 based on ELF class.

The patch follows the implementation approach previously used for MIPS.
It defines RISC-V architecture subtypes and inspects the ELF header,
namely the ELF class, to detect the right subtype.

Differential Revision: https://reviews.llvm.org/D86292