Make improvements

- Improve compatibility with Blink virtual machine
- Add non-POSIX APIs for joining threads and signal masks
- Never ever use anything except 32-bit integers for atomics
- Add some `#undef` statements to workaround `ctags` problems

libc/thread/pthread_attr_setschedparam.c

@@ -42,7 +42,6 @@
  */
 errno_t pthread_attr_setschedparam(pthread_attr_t *attr,
                                    const struct sched_param *param) {
-  if (!param) return EINVAL;
   attr->__schedparam = param->sched_priority;
   return 0;
 }