This should make objcopy -B redundant for the common case of producing
ELF output where the -O target defaults to the desired arch:mach.

	PR 24968
	* objcopy.c (copy_object): For ELF output and non-ELF input without
	arch, take arch from output file if not given by -B.  Don't
	bfd_get_arch_info when we already have iarch.