[libunwind][LoongArch] Add 64-bit LoongArch support

Defines enums for the LoongArch registers.
Adds the register class implementation for LoongArch.
Adds save and restore context functionality.

This only supports 64 bits integer and float-point register
implementation.

Fix https://github.com/llvm/llvm-project/issues/55398

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D137010