Darwin, Objective-C : Adjust objective-c symbol linkage with version.

Darwin, Objective-C : Adjust objective-c symbol linkage with version.

Earlier linkers cannot handle publicly-visible (or linker-visible)
metadata, so we need to make the output of these conditional on version.

gcc/ChangeLog:

	* config/darwin.c (darwin_globalize_label): Make a subset of
	metadate symbols global.
	(darwin_label_is_anonymous_local_objc_name): Make a subset of
	metadata symbols linker-visible.
	(darwin_override_options): Track more target OS versions, make
	the next_runtime version track this (unless it's set to 0 for
	GNU runtime).

gcc/objc/ChangeLog:

	* objc-next-runtime-abi-02.c (FIXUP_NEEDED): Rename ...
	(USE_FIXUP_BEFORE): ... to this.
	(next_runtime_02_initialize): Likewise.
	(next_runtime_abi_02_get_arg_type_list_base): Likewise.
	(next_runtime_abi_02_build_objc_method_call): Likewise.