Use gomp_map_val for OpenACC host-to-device address translation

	libgomp/
	* libgomp.h (gomp_map_val): Add prototype.
	* oacc-parallel.c (GOACC_parallel_keyed): Use gomp_map_val instead of
	open-coding device-address calculation.
	* target.c (gomp_map_val): Make global. Use OFFSET_POINTER in
	non-present case.

Co-Authored-By: Cesar Philippidis <cesar@codesourcery.com>

From-SVN: r279622