Import HSA header files from AMD

These are the same header files that exist in the Radeon Open Compute Runtime
project (as of October 2020), but they have been specially relicensed by AMD
for use in GCC.

The header files retain AMD copyright.

include/ChangeLog:

	* hsa.h: Replace whole file.
	* hsa_ext_amd.h: New file.
	* hsa_ext_image.h: New file.

libgomp/ChangeLog:

	* plugin/plugin-gcn.c: Include hsa_ext_amd.h.
	(HSA_AMD_AGENT_INFO_COMPUTE_UNIT_COUNT): Delete redundant definition.