mac: arrange for arm64 run on arm64

Makes 9term.app work again (not put you in an x86_64 jail).

mac/Plumb.app/Contents/MacOS/plumb

@@ -3,6 +3,10 @@
 if [ -e ~/.bashrc ] ; then
 	. ~/.bashrc
 fi
+arch=x86_64
+if arch -arch arm64 date >/dev/null 2>&1; then
+	arch=arm64
+fi
 PLAN9=${PLAN9:-/usr/local/plan9}
 
 bin=$PLAN9/bin
@@ -10,5 +14,5 @@ IFS=$'\n'
 
 for file in $($bin/macargv)
 do
-	$bin/macedit "$file"
+	arch -arch $arch $bin/macedit "$file"
 done