libgomp/gcn: fix/improve struct output

output.printf_data.(value union) contains text[128], which has the size
of 128 bytes, sufficient for 16 uint64_t variables; hence value_u64[2]
could be extended to value_u64[6] - sufficient for all required arguments
to gomp_target_rev.  Additionally, next_output.printf_data.(msg union)
contained msg_u64 which then is no longer needed and also caused 32bit
vs 64bit alignment issues.

libgomp/
	* config/gcn/libgomp-gcn.h (struct output):
	Remove 'msg_u64' from the union, change
	value_u64[2] to value_u64[6].
	* config/gcn/target.c (GOMP_target_ext): Update accordingly.
	* plugin/plugin-gcn.c (process_reverse_offload, console_output):
	Likewise.