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/calls/fchown.c

@@ -28,7 +28,6 @@
  * @return 0 on success, or -1 w/ errno
  * @see /etc/passwd for user ids
  * @see /etc/group for group ids
- * @syscall
  */
 int fchown(int fd, uint32_t uid, uint32_t gid) {
   /* TODO(jart): Windows? */