Define __start/__stop symbols if they are referenced by shared objects,
not if they are also defined in shared objects.
bfd/
PR ld/21964
* elflink.c (bfd_elf_define_start_stop): Check if __start and
__stop symbols are referenced by shared objects.
ld/
PR ld/21964
* testsuite/ld-elf/pr21964-4.c: New file.
* testsuite/ld-elf/shared.exp: Run pr21964-4 test on Linux.
