clang/AMDGPU: Refactor triple adjustments (#190343)
Factor this similar to the ARM case for future
expansion. The difference being -mcpu is treated as
an alias for -mcpu instead of something separately
useful.
I don't understand this mutation of the triple into
spirv64. The only test where this appears to matter
does not use -mcpu. Previously this would only match
for -mcpu, but this would change the behavior to prefer
-march before falling back to -mcpu.
