Fortran/OpenMP: Add memory routines existing for C/C++

This patch adds the Fortran interface for omp_alloc/omp_free
and the omp_target_* memory routines, which were added in
OpenMP 5.0 for C/C++ but only OpenMP 5.1 added them for Fortran.

Those functions use BIND(C), i.e. on the libgomp side, the same
interface as for C/C++ is used.

Note: By using BIND(C) in omp_lib.h, files including this file
no longer compiler with -std=f95 but require at least -std=f2003.

libgomp/ChangeLog:

	* omp_lib.f90.in (omp_alloc, omp_free, omp_target_alloc,
	omp_target_free. omp_target_is_present, omp_target_memcpy,
	omp_target_memcpy_rect, omp_target_associate_ptr,
	omp_target_disassociate_ptr): Add interface.
	* omp_lib.h.in (omp_alloc, omp_free, omp_target_alloc,
	omp_target_free. omp_target_is_present, omp_target_memcpy,
	omp_target_memcpy_rect, omp_target_associate_ptr,
	omp_target_disassociate_ptr): Add interface.
	* testsuite/libgomp.fortran/alloc-1.F90: Remove local
	interface block for omp_alloc + omp_free.
	* testsuite/libgomp.fortran/alloc-4.f90: Likewise.
	* testsuite/libgomp.fortran/refcount-1.f90: New test.
	* testsuite/libgomp.fortran/target-12.f90: New test.