Use the correct stat command for macos and linux

bin/padto

@@ -2,6 +2,10 @@
 
 totalsize=$1
 outfile=$2
-filesize=`stat -f "%z" "$outfile"`
+
+[[ "$OSTYPE" == "linux-gnu" ]] \
+    && filesize=$(stat -c "%s" "$outfile") \
+    || filesize=$(stat -f "%z" "$outfile")
+
 padsize=$((512-$filesize))
 dd if=/dev/zero bs=1 count=$padsize >> "$outfile"