Objective-C++, Darwin : Make metadata 'extern "C"'.

Objective-C++, Darwin : Make metadata 'extern "C"'.

For current system toolchains NeXT runtime metadata symbols are not
mangled for Objective-C++ (i.e. they are considered to be
'extern "C"').

This change becomes essential when we start to emit metadata refs
as hidden and weak which is required by later editions of the runtime
and linkers.

gcc/objc/ChangeLog:

	* objc-runtime-shared-support.c (start_var_decl): Make the
	decl_assembler_name follow the metadata name for C++ on NeXT
	runtime platforms.