Objective-C, Darwin : Make protocols private extern and weak.

Newer linkers want protocols to be private-extern and weak.
This is conditional on the runtime version.

gcc/objc/ChangeLog:

	* objc-next-runtime-abi-02.c
	(create_global_decl): Update to allow the caller to
	specify declaration or definition.
	(create_hidden_decl): Likewise.
	(next_runtime_abi_02_protocol_decl): Make the symbol
	weak and hidden for later OS runtime versions.
	(build_v2_protocol_list_address_table): Likewise.