Make more improvements to threads and mappings

- NetBSD should now have faster synchronization
- POSIX barriers may now be shared across processes
- An edge case with memory map tracking has been fixed
- Grand Central Dispatch is no longer used on MacOS ARM64
- POSIX mutexes in normal mode now use futexes across processes

libc/thread/pthread_barrierattr_init.c

@@ -24,6 +24,6 @@
  * @return 0 on success, or error on failure
  */
 errno_t pthread_barrierattr_init(pthread_barrierattr_t *attr) {
-  *attr = 0;
+  *attr = PTHREAD_PROCESS_PRIVATE;
   return 0;
 }