OpenMP, libgomp: Add new runtime routine omp_get_mapped_ptr.

This patch adds the OpenMP runtime routine "omp_get_mapped_ptr" which was
introduced in OpenMP 5.1.

gcc/ChangeLog:

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

libgomp/ChangeLog:

	* libgomp.map: Added omp_get_mapped_ptr.
	* libgomp.texi: Tagged omp_get_mapped_ptr as supported.
	* omp.h.in: Added omp_get_mapped_ptr.
	* omp_lib.f90.in: Added interface for omp_get_mapped_ptr.
	* omp_lib.h.in: Likewise.
	* target.c (omp_get_mapped_ptr): Added implementation of
	omp_get_mapped_ptr.
	* testsuite/libgomp.c-c++-common/get-mapped-ptr-1.c: New test.
	* testsuite/libgomp.c-c++-common/get-mapped-ptr-2.c: New test.
	* testsuite/libgomp.c-c++-common/get-mapped-ptr-3.c: New test.
	* testsuite/libgomp.c-c++-common/get-mapped-ptr-4.c: New test.
	* testsuite/libgomp.fortran/get-mapped-ptr-1.f90: New test.
	* testsuite/libgomp.fortran/get-mapped-ptr-2.f90: New test.
	* testsuite/libgomp.fortran/get-mapped-ptr-3.f90: New test.
	* testsuite/libgomp.fortran/get-mapped-ptr-4.f90: New test.