Only support ifunc in nptl/pt-vfork.c

	* nptl/pt-vfork.c: Error if !HAVE_IFUNC.
	[!HAVE_IFUNC] (vfork_compat): Remove.
	[!HAVE_IFUNC] (DEFINE_VFORK): Remove.
Only provide non-default symbols in libpthread for vfork

	* nptl/pt-vfork.c (vfork_resolve): Rename from vfork_ifunc.
	(vfork, __vfork): Define via compat_symbol.