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/fchmod.c

@@ -40,7 +40,6 @@
  * @errors ENOSYS
  * @asyncsignalsafe
  * @see chmod()
- * @syscall
  */
 int fchmod(int fd, uint32_t mode) {
   /* TODO(jart): Windows */