[OpenACC] Repair/restore 'is_tgt_unmapped' checking

[OpenACC] Repair/restore 'is_tgt_unmapped' checking

	libgomp/
	* oacc-mem.c (goacc_exit_datum): Repair 'is_tgt_unmapped'
	checking.
	(acc_unmap_data, goacc_exit_data_internal): Restore
	'is_tgt_unmapped' checking.
	* testsuite/libgomp.oacc-c-c++-common/struct-refcount-1.c: New
	file.
	* testsuite/libgomp.oacc-fortran/deep-copy-6.f90: Adjust.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-1-1.f90: Likewise.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-2-1.f90: Likewise.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-2-2.f90: Likewise.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-3-1.f90: Likewise.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-4-1.f90: Likewise.

Co-Authored-By: Julian Brown <julian@codesourcery.com>