PR22374, PowerPC unnecessary PLT entries

PR22374, PowerPC unnecessary PLT entries

We don't need a PLT entry when function pointer initialization in a
read/write section is the only reference to a given function symbol.
This patch prevents the unnecessary PLT entry, and ensures no dynamic
relocs are emitted when UNDEFWEAK_NO_DYNAMIC_RELOC says so.

bfd/
	PR 22374
	* elf32-ppc.c (ppc_elf_adjust_dynamic_symbol): Don't create a plt
	entry when just a dynamic reloc can serve.  Ensure no dynamic
	relocations when UNDEFWEAK_NO_DYNAMIC_RELOC by setting non_got_ref.
	Expand and move the non_got_ref comment.
	* elf64-ppc.c (ppc64_elf_adjust_dynamic_symbol): Likewise.
ld/
	* testsuite/ld-powerpc/ambiguousv2.d: Remove FIXME.