Speed up unnamed POSIX semaphores

When sem_wait() used its futexes it would always use process shared mode
which can be problematic on platforms like Windows, where that causes it
to use the slow futex polyfill. Now when sem_init() is called in private
mode that'll be passed along so we can use a faster WaitOnAddress() call

test/posix/unnamed_semaphore_test.c

@@ -0,0 +1,35 @@
+#include <pthread.h>
+#include <semaphore.h>
+
+#define THREADS    10
+#define ITERATIONS 100000
+
+int g_count;
+sem_t g_sem;
+
+void *worker(void *arg) {
+  for (int i = 0; i < ITERATIONS; ++i) {
+    if (sem_wait(&g_sem))
+      exit(6);
+    ++g_count;
+    if (sem_post(&g_sem))
+      exit(7);
+  }
+  return 0;
+}
+
+int main(int argc, char *argv[]) {
+  pthread_t th[THREADS];
+  if (sem_init(&g_sem, 0, 1))
+    return 1;
+  for (int i = 0; i < THREADS; ++i)
+    if (pthread_create(&th[i], 0, worker, 0))
+      return 2;
+  for (int i = 0; i < THREADS; ++i)
+    if (pthread_join(th[i], 0))
+      return 3;
+  if (g_count != THREADS * ITERATIONS)
+    return 4;
+  if (sem_destroy(&g_sem))
+    return 5;
+}