amdgcn: Match new XNACK defaults in mkoffload

The patch that disabled XNACK by default for ISA other than gfx90a was missing
the matching mkoffload changes.  This patch should fix offload.

gcc/ChangeLog:

	* config/gcn/mkoffload.cc (TEST_XNACK_UNSET): New.
	(elf_flags): Remove XNACK from the default value.
	(main): Set a default XNACK according to the arch.