[build] Fix signed/unsigned division in util/zbin.c

Commit b149a99 ([build] Round up SUBx deltas) introduced a
signed/unsigned issue that affects gPXE images built on 32-bit hosts.
The zbin fixup utility performed an unsigned division, which led to
.usb images with an incorrect number of sectors to load.

The issue snuck by on 64-bit hosts since uint32_t is promoted to long.
On 32-bit hosts it is promoted to unsigned long.

Modified-by: Michael Brown <mcb30@etherboot.org>
Signed-off-by: Michael Brown <mcb30@etherboot.org>

src/util/zbin.c

 				    size_t datasize ) {
 	size_t offset = subtract->offset;
 	void *target;
-	long delta;
+	signed long raw_delta;
+	signed long delta;
 	unsigned long old;
 	unsigned long new;
 
 	}
 
 	target = ( output->buf + offset );
-	delta = ( ( align ( output->len, subtract->divisor ) -
-		    align ( input->len, subtract->divisor ) )
-		  / subtract->divisor );
+	raw_delta = ( align ( output->len, subtract->divisor ) -
+		      align ( input->len, subtract->divisor ) );
+	delta = ( raw_delta / ( ( signed long ) subtract->divisor ) );
 
 	switch ( datasize ) {
 	case 1: {