blindly implemented exit system call

common.h

@@ -22,6 +22,7 @@ typedef uint32_t vaddr_t;
 #define PAGE_SIZE 4096
 #define SYS_PUTCHAR 1
 #define SYS_GETCHAR 2
+#define SYS_EXIT    3
 
 void *memset(void *buf, char c, size_t n);
 void *memcpy(void *dst, const void *src, size_t n);