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/intrin/pthreadlock.c

@@ -20,6 +20,10 @@
 
 pthread_spinlock_t _pthread_lock_obj;
 
+void _pthread_init(void) {
+  (void)pthread_spin_init(&_pthread_lock_obj, 0);
+}
+
 void _pthread_lock(void) {
   pthread_spin_lock(&_pthread_lock_obj);
 }