Currently, we don't at all evaluate 'copyfrom' for 'GOMP_MAP_STRUCT' entries.
Fix this by copying/adapting the corresponding non-'GOMP_MAP_STRUCT' code.

	libgomp/
	* oacc-mem.c (goacc_exit_data_internal) <GOMP_MAP_STRUCT>:
	Evaluate 'copyfrom' individually for each entry.
	* testsuite/libgomp.oacc-c-c++-common/struct-1.c: Update.

(cherry picked from commit 2c838a3e4ea06c69c856d074ae5b0400e08ae3c2)