[AMDGPU] Implement wave64 DWARF register mapping

Summary:
Implement the DWARF register mapping described in
llvm/docs/AMDGPUUsage.rst

This is currently limited to wave64 VGPRs/AGPRs.

This also includes some minor changes in AMDGPUInstPrinter,
AMDGPUMCTargetDesc, and AMDGPUAsmParser to make generating CFI assembly
text and ELF sections possible to ease testing, although complete CFI
support is not yet implemented.

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74915