libgomp: Add '__attribute__((unused))' to variables used only in 'assert(...)'

Without this attribute, the building process will fail if GCC is configured
with 'CFLAGS=-DNDEBUG'.

libgomp/ChangeLog:

	* oacc-mem.c (acc_unmap_data, goacc_exit_datum_1, find_group_last,
	goacc_enter_data_internal): Add '__attribute__((unused))'.
	* target.c (gomp_unmap_vars_internal): Likewise.