PR22431, powerpc64 ld segfault when .plt discarded

PR22431, powerpc64 ld segfault when .plt discarded

The fix for the PR is to not use input_section->output_section->owner
to get to the output bfd, but use the output bfd directly since it is
available nowadays in struct bfd_link_info.

I thought it worth warning when non-empty dynamic sections are
discarded too, which meant a tweak to one of the ld tests to avoid the
warning.

bfd/
	PR 22431
	* elf64-ppc.c (ppc64_elf_size_dynamic_sections): Warn on discarding
	non-empty dynamic section.
	(ppc_build_one_stub): Take elf_gp from output bfd, not output
	section owner.
	(ppc_size_one_stub, ppc64_elf_next_toc_section): Likewise.
ld/
	* testsuite/ld-elf/note-3.t: Don't discard .got.