Handle -z unique/-z nounique in ld

Handle -z unique/-z nounique in ld

Add (or suppress) a DT_GNU_FLAGS_1 dynamic section
with a bit flag value of DF_GNU_1_UNIQUE.

bfd/
	* elflink.c (bfd_elf_size_dynamic_sections): Call
	_bfd_elf_add_dynamic_entry to add a DT_GNU_FLAGS_1 section.
include/
	* bfdlink.h (struct bfd_link_info): New field gnu_flags_1.
ld/
	* emultempl/elf.em (gld${EMULATION_NAME}_handle_option):
	Parse -z unique / -z nounique options.