Only give FDE encoding warnings if --eh-frame-hdr was specified.

Only give FDE encoding warnings if --eh-frame-hdr was specified.

This bug was observed on nios2-linux-gnu with some C++ programs
linked with -pie or -shared.  The nios2 ABI doesn't include appropriate
relocations in this instance and GCC is also being patched not to pass
--eh-frame-hdr to the linker in those cases.

2019-12-05  Sandra Loosemore  <sandra@codesourcery.com>

        bfd/
        * elf-eh-frame.c (_bfd_elf_discard_section_eh_frame): Make
        FDE encoding warning conditional.