unix: support generating netbsd/arm64 files in mkall.sh

CL 155738 added the generated files but didn't update mkall.sh. Do so
now.

Change-Id: I8681ef175547750bea94b06827ef5f4512dc6de3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/174860
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

unix/mkall.sh

@@ -146,6 +146,12 @@ netbsd_arm)
 	# API consistent across platforms.
 	mktypes="GOARCH=$GOARCH go tool cgo -godefs -- -fsigned-char"
 	;;
+netbsd_arm64)
+	mkerrors="$mkerrors -m64"
+	mksyscall="go run mksyscall.go -netbsd"
+	mksysnum="go run mksysnum.go 'http://cvsweb.netbsd.org/bsdweb.cgi/~checkout~/src/sys/kern/syscalls.master'"
+	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
+	;;
 openbsd_386)
 	mkerrors="$mkerrors -m32"
 	mksyscall="go run mksyscall.go -l32 -openbsd"