openacc: Helper functions for enter/exit data using single mapping

This patch factors out the parts of goacc_enter_datum and
goacc_exit_datum that can be shared with goacc_enter_data_internal
and goacc_exit_data_internal respectively (in the next patch),
without overloading function return values or complicating code paths
unnecessarily.

2020-07-10  Julian Brown  <julian@codesourcery.com>
	    Thomas Schwinge  <thomas@codesourcery.com>

libgomp/
	* oacc-mem.c (goacc_map_var_existing): New function.
	(goacc_enter_datum): Use above function.
	(goacc_exit_datum_1): New function.
	(goacc_exit_datum): Use above function.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>