Add common ifunc-init.h header

This patch moves the generic definition from x86_64 init-arch
to a common header ifunc-init.h.  No functional changes is expected.

Checked on a x86_64-linux-gnu build.

	* sysdeps/generic/ifunc-init.h: New file.
	* sysdeps/x86/init-arch.h: Use generic ifunc-init.h.