[AMDGPU] Use a consistent DwarfEH register flavour (#84513)

Previously, we always used the wave64 encodings for EH registers
regardless of whether we were compiling for wave32, which seems wrong.
We don't seem to use the EH registers, so this commit is mostly just
about papering over code that converts from non-EH dwarf registers to
LLVM registers while claiming they are EH dwarf registers. That kind of
code should be okay on any non-darwin target (since darwin is the only
target that uses a different encoding for EH registers).