recognise arm64

Index: ld/configure.tgt
--- ld/configure.tgt.orig
+++ ld/configure.tgt
@@ -98,6 +98,9 @@ aarch64-*-freebsd*)	targ_emul=aarch64fbsd
 aarch64-*-fuchsia*)	targ_emul=aarch64elf
 			targ_extra_emuls="aarch64elfb armelf armelfb"
 			;;
+aarch64-*-openbsd*)	targ_emul=aarch64obsd
+			targ_extra_emuls="aarch64obsdb aarch64elf"
+			;;
 aarch64_be-*-linux-gnu_ilp32)
 			targ_emul=aarch64linux32b
 			targ_extra_libpath="aarch64linuxb aarch64linux aarch64linux32 armelfb_linux_eabi armelf_linux_eabi"
