Harden powerpc64 elf_machine_fixup_plt

Harden powerpc64 elf_machine_fixup_plt

IFUNC is difficult to correctly implement on any target needing a GOT
to support position independent code, due to the dependency on order
of dynamic relocations.  ld.so should be changed to apply IFUNC
relocations last, globally, because without that it is actually
impossible to write an IFUNC resolver in C that works in all
situations.  Case in point, vfork in libpthread.so is an IFUNC with
the resolver returning &__libc_vfork.  (system and fork are similar.)
If another shared library, libA say, uses vfork then it is quite
possible that libpthread.so hasn't been dynamically relocated before
the unfortunate libA is dynamically relocated.  In that case the GOT
entry for &__libc_vfork is still zero, so the IFUNC resolver returns
NULL.  LD_BIND_NOW=1 results in libA PLT dynamic relocations being
applied using this NULL value and ld.so segfaults.

This patch hardens ld.so to not segfault on a NULL from an IFUNC
resolver.  It also fixes a problem with undefined weak.  If you leave
the plt entry as-is for undefined weak then if the entry is ever
called it will loop in ld.so rather than segfaulting.

	* sysdeps/powerpc/powerpc64/dl-machine.h (elf_machine_fixup_plt):
	Don't segfault if ifunc resolver returns a NULL.  Do set plt to
	zero for undefined weak.
	(elf_machine_plt_conflict): Similarly.