libgomp: with USM, init 'link' variables with host address

If requires unified_shared_memory or self_maps is set, make
'declare target link' variables to point initially to the host pointer.

libgomp/ChangeLog:

	* target.c (gomp_load_image_to_device): For requires
	unified_shared_memory, update 'link' vars to point to the host var.
	* testsuite/libgomp.c-c++-common/target-link-3.c: New test.
	* testsuite/libgomp.c-c++-common/target-link-4.c: New test.