OpenMP: Fix omp_get_device_from_uid, minor cleanup

In Fortran, omp_get_device_from_uid can also accept substrings, which are
then not NUL terminated.  Fixed by introducing a fortran.c wrapper function.
Additionally, in case of a fail the plugin functions now return NULL instead
of failing fatally such that a fall-back UID is generated.

gcc/ChangeLog:

	* omp-general.cc (omp_runtime_api_procname): Strip "omp_" from
	string; move get_device_from_uid as now a '_' suffix exists.

libgomp/ChangeLog:

	* fortran.c (omp_get_device_from_uid_): New function.
	* libgomp.map (GOMP_6.0): Add it.
	* oacc-host.c (host_dispatch): Init '.uid' and '.get_uid_func'.
	* omp_lib.f90.in: Make it used by removing bind(C).
	* omp_lib.h.in: Likewise.
	* target.c (omp_get_device_from_uid): Ensure the device is initialized.
	* plugin/plugin-gcn.c (GOMP_OFFLOAD_get_uid): Add function comment;
	return NULL in case of an error.
	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_get_uid): Likewise.
	* testsuite/libgomp.fortran/device_uid.f90: Update to test substrings.