S/390: z13 use GNU attribute to indicate vector ABI

S/390: z13 use GNU attribute to indicate vector ABI

bfd/
	* elf-s390-common.c (elf_s390_merge_obj_attributes): New function.
	* elf32-s390.c (elf32_s390_merge_private_bfd_data): Call
	elf_s390_merge_obj_attributes.
	* elf64-s390.c (elf64_s390_merge_private_bfd_data): New function.

binutils/
	* readelf.c (display_s390_gnu_attribute): New function.
	(process_s390_specific): New function.
	(process_arch_specific): Call process_s390_specific.

gas/
	* doc/as.texinfo: Document Tag_GNU_S390_ABI_Vector.

include/elf/
	* s390.h: Define Tag_GNU_S390_ABI_Vector.