libgomp.texi: Document supported OpenMP 'interop' types for nvptx and gcn

libgomp.texi: Document supported OpenMP 'interop' types for nvptx and gcn

Note that this commit also updates the API interface to OpenMP 6.0;
while 5.1 and 5.2 use 'int *' for the the ret_code argument,
OpenMP 6.0 changed this to omp_interop_rc_t *; this enum also exists in
OpenMP 5.1. However, C++ does not like this change such that unless NULL
is passed (i.e. the argument is ignored), OpenMP 5.x and 6.x are not
compatible.

Note that GCC's omp.h already follows OpenMP 6.0 and is now in sync with
the documentation.

libgomp/ChangeLog:

	* libgomp.texi (OpenMP 5.1): Add @ref to offload-target specifics
	for 'interop'.
	(OpenMP 6.0): Mark dispatch's interop clause as implemented.
	(omp_get_interop_int, omp_get_interop_str,
	omp_get_interop_ptr, omp_get_interop_type_desc): Add @ref to
	Offload-Target Specifics; change ret_code argument type to
	'omp_interop_rc_t *'.
	(Offload-Target Specifics): Document the supported OpenMP
	interop foreign runtimes on AMD and Nvidia GPUs.