Add user/div0.

Makefile

@@ -47,8 +47,8 @@ debug-nox: image.bin
 		-ex "break _start" \
 		-ex "continue"
 
-fs.img: kernel.bin tools/mkfs user/false user/greet
-	tools/mkfs $@ $< user/false user/greet
+fs.img: kernel.bin tools/mkfs user/false user/greet user/div0
+	tools/mkfs $@ $< user/false user/greet user/div0
 
 LDFLAGS=-m elf_i386
 

user/div0.c

@@ -0,0 +1,5 @@
+int main(void) {
+    volatile int x = 1, y = 0;
+    x /= y;
+    return 0;
+}