Added new metadata fields for code object version 5. Defined various
offset and size constants.
Here are some salient differences in cov5 and cov4:
- Size of Implicit KernArgs field is now 256 (used to be 56 in cov4).
- Different metadata fields have a well defined starting location and size (cov4 used to have 7 fields of 8 bytes each).
- Cov5 uses ImplictArg ptr instead of dispatch ptr to query workgroup_size_x so __builtin_amdgcn_workgroup_size_x() does not work.