OpenMP: Add omp_default_device named constant [PR119677]

OpenMP: Add omp_default_device named constant [PR119677]

OpenMP TR 14 (OpenMP 6.1) adds omp_default_device < -1 as
named constant alongside omp_initial_device and omp_default_device.

GCC supports it already internally via GOMP_DEVICE_DEFAULT_OMP_61,
but this patch now adds the omp_default_device enum/PARAMETER to
omp.h / omp_lib.

Note that PR119677 requests some cleanups, which still have to be
done.

	PR libgomp/119677

gcc/fortran/ChangeLog:

	* intrinsic.texi (OpenMP Modules): Add omp_default_device.
	* openmp.cc (gfc_resolve_omp_context_selector): Accept
	omp_default_device as conforming device number.

libgomp/ChangeLog:

	* omp.h.in (omp_default_device): New enum value.
	* omp_lib.f90.in: New parameter.
	* omp_lib.h.in: Likewise
	* target.c (gomp_get_default_device): New. Split off from ...
	(resolve_device): ... here; call it.
	(omp_target_alloc, omp_target_free, omp_target_is_present,
	omp_target_memcpy_check, omp_target_memset, omp_target_memset_async,
	omp_target_associate_ptr, omp_get_mapped_ptr,
	omp_target_is_accessible, omp_pause_resource,
	omp_get_uid_from_device): Handle omp_default_device.
	* testsuite/libgomp.c/device_uid.c: Likewise.
	* testsuite/libgomp.fortran/device_uid.f90: Likewise.
	* testsuite/libgomp.c-c++-common/omp-default-device.c: New test.
	* testsuite/libgomp.fortran/omp-default-device.f90: New test.