Implement _Exit(3).

Change-Id: Ida6ac844cc87d38c9645b197dd8188bb73e27dbe
diff --git a/libc/include/stdlib.h b/libc/include/stdlib.h
index 5dbcb3d..2c797c5 100644
--- a/libc/include/stdlib.h
+++ b/libc/include/stdlib.h
@@ -41,8 +41,9 @@
 #define EXIT_FAILURE 1
 #define EXIT_SUCCESS 0
 
-extern __noreturn void exit(int);
 extern __noreturn void abort(void);
+extern __noreturn void exit(int);
+extern __noreturn void _Exit(int);
 extern int atexit(void (*)(void));
 
 extern char *getenv(const char *);