OpenMP/Fortran: Fix handling of 'declare target' with 'link' clause [PR115559]

OpenMP/Fortran: Fix handling of 'declare target' with 'link' clause [PR115559]

Contrary to a normal 'declare target', the 'declare target link' attribute
also needs to set node->offloadable and push the offload_vars in the front end.

Linked variables require that the data is mapped. For module variables, this
can happen anywhere. For variables in an external subprograms or the main
programm, this can only happen in the either that program itself or in an
internal subprogram. - Whether a variable is just normally mapped or linked then
becomes relevant if a device routine exists that can access that variable,
i.e. an internal procedure has then to be marked as declare target.

	PR fortran/115559

gcc/fortran/ChangeLog:

	* trans-common.cc (build_common_decl): Add 'omp declare target' and
	'omp declare target link' variables to offload_vars.
	* trans-decl.cc (add_attributes_to_decl): Likewise; update args and
	call decl_attributes.
	(get_proc_pointer_decl, gfc_get_extern_function_decl,
	build_function_decl): Update calls.
	(gfc_get_symbol_decl): Likewise; move after 'DECL_STATIC (t)=1'
	to avoid errors with symtab_node::get_create.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/declare-target-link.f90: New test.