[gcn] Fix the output amdhsa.version

The amdhsa.version depends on the code object version; while V3 had 1.0,
V4 has 1.1 and V5 (and V6) have 1.2. GCC used 1.0 but generated since
a while either V4 or, with -march=gfx...-generic, V6. Now it uses the
proper version again.

gcc/ChangeLog:

	* config/gcn/gcn.cc (gcn_hsa_declare_function_name): Update
	'amdhsa.version' output to match used code version.
	* config/gcn/gen-gcn-device-macros.awk: Add a comment to
	crosslink.