PR 1150

PR 1150
* elf-bfd.h (struct elf_backend_data): New field 'elf_backend_ignore_undef_symbol'.
* elfxx-target.h (elf_backend_ignore_undef_symbol): Define to NULL if not
  already defined.
  (elfNN_bed): Initialise the elf_backend_ignore_undef_symbol field.
* elfxx-mips.c (_bfd_mips_elf_ignore_undef_symbol): New function.
* elfxx-mips.h (elf_backend_ignore_undef_symbol): Define and prototype.
* elflink.c (elf_link_output_extsym): Check elf_backend_ignore_undef_symbol
  before reporting an undefined symbol in a shared library.