Make locks more reliable

This change switches most of the core locks to be re-entrant, in order
to reduce the chance of deadlocking code that does, clever things with
asynchronous signal handlers. This change implements it it in pthreads
so we're one step closer to having a standardized threading primitives

libc/runtime/printmaps.c

@@ -24,7 +24,7 @@
  * Prints memory mappings to stderr.
  */
 void __print_maps(void) {
-  _spinlock(&_mmi.lock);
+  __mmi_lock();
   PrintMemoryIntervals(2, &_mmi);
-  _spunlock(&_mmi.lock);
+  __mmi_unlock();
 }