Fix seg-fault when generating an empty DLL with LTO enabled.

ld   PR 29998
     * pe-dll.c (generate_reloc): Handle sections
     with no assigned output section.
     Terminate early of there are no relocs to put
     in the .reloc section.
     (pe_exe_fill_sections): Do not emit an empty
     .reloc section.

bfd  * cofflink.c (_bfd_coff_generic_relocate_section):
     Add an assertion that the output section is set
     for defined, global symbols.