AMDGPU: Respect compute ABI attributes with unknown OS

Unfortunately Mesa is still using amdgcn-- as the triple for OpenGL,
so we still have the awkward unknown OS case to deal with. Previously
if the HSA ABI intrinsics appeared, we we would not add the ABI
registers to the function. We would emit an error later, but we still
need to produce some compile result. Start adding the registers to any
compute function, regardless of the OS. This keeps the internal state
more consistent, and will help avoid numerous test crashes in a future
patch which starts assuming the ABI inputs are present on functions by
default.