Objective-C, Darwin : Pick up super refs directly.

Objective-C, Darwin : Pick up super refs directly.

The current code assumed that super refs could be computed
indirectly, i.e. that the metadata generated by the compiler
was immutable by the runtime. This does not always hold
(it depends on the NeXT runtime version).  So, compute super
refs directly.

gcc/objc/ChangeLog:

	* objc-next-runtime-abi-02.c
	(objc_get_superclass_ref_decl): Split this code out.
	(next_runtime_abi_02_get_class_super_ref): Compute
	super refs using the objc_get_superclass_ref_decl().
	(next_runtime_abi_02_get_category_super_ref): Likewise.