Prepare powerpc64 for late check_relocs

Prepare powerpc64 for late check_relocs

check_relocs was setting up some data used by the --gc-sections
gc_mark_hook.  If we change ld to run check_relocs after gc_sections
that data needs to be set up elsewhere.  Done by this patch in the
backend check_directives function (ppc64_elf_before_check_relocs).

	* elf64-ppc.c (ppc64_elf_before_check_relocs): Set sec_type for
	.opd whenever .opd is present and non-zero size.  Move code
	setting abiversion to/from output file earlier.  Only set
	u.opd.func_sec when --gc-sections.  Read relocs and set up
	u.opd.func_sec values here..
	(ppc64_elf_check_relocs): ..rather than here.  Simplify opd
	section tests.
	(ppc64_elf_edit_opd): Don't set sec_type for .opd here.