EM_AARCH64 is defined as a constant, but the corresponding entry in
machineStrings is missing. Add it.

Change-Id: I6506404386efe608877095e635a290bbc0686215
Reviewed-on: https://go-review.googlesource.com/106035
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>