.gitignore: ignore ARM64 build products

Change-Id: I56297aac4ee282fd117ec525b88dee4769477111
Reviewed-on: https://go-review.googlesource.com/7560Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

.gitignore

 .DS_Store
-*.[5689ao]
-*.a[5689o]
+*.[56789ao]
+*.a[56789o]
 *.so
 *.pyc
 ._*
 .nfs.*
-[5689a].out
+[56789a].out
 *~
 *.orig
 *.rej