OpenMP, libgomp: Add new runtime routine omp_target_is_accessible.

OpenMP, libgomp: Add new runtime routine omp_target_is_accessible.

gcc/ChangeLog:

	* omp-low.cc (omp_runtime_api_call): Added target_is_accessible to
	omp_runtime_apis array.

libgomp/ChangeLog:

	* libgomp.map: Added omp_target_is_accessible.
	* libgomp.texi: Tagged omp_target_is_accessible as supported.
	* omp.h.in: Added omp_target_is_accessible.
	* omp_lib.f90.in: Added interface for omp_target_is_accessible.
	* omp_lib.h.in: Likewise.
	* target.c (omp_target_is_accessible): Added implementation of
	omp_target_is_accessible.
	* testsuite/libgomp.c-c++-common/target-is-accessible-1.c: New test.
	* testsuite/libgomp.fortran/target-is-accessible-1.f90: New test.

(cherry picked from commit 4043f53cb4a3541f7a6e4f4132419f78ab7ec4f7)