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/intrin/pthread_yield_np.c

@@ -32,7 +32,7 @@ void sys_sched_yield(void);
 int pthread_yield_np(void) {
   if (IsXnuSilicon()) {
     __syslib->__pthread_yield_np();
-  } else if (IsOpenbsd() || IsNetbsd()) {
+  } else if (IsOpenbsd()) {
     // sched_yield() is punishingly slow on OpenBSD
     // it's ruinously slow it'll destroy everything
     pthread_pause_np();