Fix fork locking on win32 (#1141)

* Fix fork locking on win32

- __enable_threads / set __threaded in __proc_setup as threads are required for
  win32 subprocess management
- move mmi/fds locking out of pthread_atfork.c into fork.c so it's done anytime
  __threaded is set instead of being dependent of pthreads
- explicitly yoink _pthread_onfork_prepare, _pthread_onfork_parent, and
  _pthread_onfork_child in pthread_create.c so they are linked in in-case they
  are separated from _pthread_atfork

Big Thanks to @dfyz for help with locating the issue, testing, and devising a fix!

* fix child processes not being able to open files, initialize all necessary locks on fork

libc/proc/proc.c

@@ -233,6 +233,7 @@ static textwindows dontinstrument uint32_t __proc_worker(void *arg) {
  * Lazy initializes process tracker data structures and worker.
  */
 static textwindows void __proc_setup(void) {
+  __enable_threads();
   __proc.onbirth = CreateEvent(0, 0, 0, 0);     // auto reset
   __proc.haszombies = CreateEvent(0, 1, 0, 0);  // manual reset
   __proc.thread = CreateThread(0, 65536, __proc_worker, 0,