Implement more security stuff

- Support deterministic stacks on OpenBSD
- Support OpenBSD system call origin verification
- Fix overrun by one in chibicc string token allocator
- Get all chibicc tests passing under Address Sanitizer

libc/runtime/stackchkfail.c

@@ -29,7 +29,7 @@
 /**
  * Aborts program under enemy fire to avoid being taken alive.
  */
-void __stack_chk_fail(void) {
+textsyscall void __stack_chk_fail(void) {
   size_t len;
   const char *msg;
   int64_t ax, cx, si;