hppa64: get_opd in elf64_hppa_mark_exported_functions

late_size_sections is too late to be creating a dynamic section, as it
won't be mapped to an output section.  So don't pretend that this will
do anything useful, and instead omit the traversal if opd_sec is NULL.
This bug showed up when using a linker-created dynobj.

	* elf64-hppa.c (elf64_hppa_mark_exported_functions): Don't
	attempt to create .opd here.
	(elf64_hppa_late_size_sections): Only mark exported functions
	for space in .opd if .opd exists.
	(elf64_hppa_finalize_dynreloc): Make the test for a discared
	other_rel_sec robust.