ARM64 doesn't have the old assembler.

Change-Id: I9253271029440e2b7f2813d3e98a7d2e7a65bfbc
Reviewed-on: https://go-review.googlesource.com/7045Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>