[gold commit] Place copy relocations in .dynbss when target is read-only

[Cary Coutant]

> 2016-12-27  Cary Coutant  <ccoutant@gmail.com>
>
> gold/
>         PR ld/20995
>         * copy-relocs.cc (Copy_relocs::make_copy_reloc): Use .dynbss for
>         read-only data when linking with -z relro.
>         * copy-relocs.h (Copy_relocs::dynrelro_): New data member.
>         * testsuite/Makefile.am (copy_test_relro): New test case.
>         * testsuite/Makefile.in: Regenerate.
>         * testsuite/copy_test_relro.cc: New source file.
>         * testsuite/copy_test_relro_1.cc: New source file.

Also backported to binutils-2_28-branch.

-cary