Place copy relocations in .dynbss when target is read-only.

Place copy relocations in .dynbss when target is read-only.

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.