Set SEC_ELF_REVERSE_COPY earlier

For the sake of DT_RELR.

bfd/
	* elflink.c (elf_link_input_bfd): Don't set SEC_ELF_REVERSE_COPY
	here.  Move sanity checks to reverse copying code.
ld/
	* ldlang.c (lang_add_section): Set SEC_ELF_REVERSE_COPY for
	.ctors/.dtors in .init_array/.fini_array.