[AMDGPU] Introduce ABI occupancy for object linking
This PR introduces ABI occupancy as the contract used to compile functions under
object linking. The default is derived from the occupancy needed for a 1024
workitem workgroup, can be overridden with `-amdgpu-abi-waves-per-eu`, and can
be overridden per function by `amdgpu-flat-work-group-size` or tightened by an
accepted `amdgpu-waves-per-eu` hint.
The backend emits the selected occupancy in `.amdgpu.info` and uses it to
enforce the object linking register budget.
