Make undefined symbols in allocate_dynrelocs dynamic

Make undefined symbols in allocate_dynrelocs dynamic

..if they have dynamic relocs.  An undefined symbol in a PIC object
that finds no definition ought to become dynamic in order to support
--allow-shlib-undefined, but there is nothing in the generic ELF
linker code to do this if the reference isn't via the GOT or PLT.  (An
initialized function pointer is an example.)  So it falls to backend
code to ensure the symbol is made dynamic.

	PR 21988
	* elf64-ppc.c (ensure_undef_dynamic): Rename from
	ensure_undefweak_dynamic.  Handle undefined too.
	* elf32-ppc.c (ensure_undef_dynamic): Likewise.
	* elf32-hppa.c (ensure_undef_dynamic): Likewise.
	(allocate_dynrelocs): Discard undefined non-default visibility
	relocs first.  Make undefined syms dynamic.  Tidy goto.