Cleanup: Use static initializers for pthread objects.

Once upon a time, POSIX specified that these static initializers could only be used for global variables, but apparently essentially all implementations have always supported these initializers for local variables as well, and POSIX recently enshrined this as a requirement.

c++/src/kj/mutex.c++

@@ -374,9 +374,7 @@ void Once::reset() {
     } \
   }
 
-Mutex::Mutex() {
-  KJ_PTHREAD_CALL(pthread_rwlock_init(&mutex, nullptr));
-}
+Mutex::Mutex(): mutex(PTHREAD_RWLOCK_INITIALIZER) {}
 Mutex::~Mutex() {
   KJ_PTHREAD_CLEANUP(pthread_rwlock_destroy(&mutex));
 }
@@ -488,9 +486,9 @@ void Mutex::lockWhen(Predicate& predicate) {
   }
 }
 
-Once::Once(bool startInitialized): state(startInitialized ? INITIALIZED : UNINITIALIZED) {
-  KJ_PTHREAD_CALL(pthread_mutex_init(&mutex, nullptr));
-}
+Once::Once(bool startInitialized)
+    : state(startInitialized ? INITIALIZED : UNINITIALIZED),
+      mutex(PTHREAD_MUTEX_INITIALIZER) {}
 Once::~Once() {
   KJ_PTHREAD_CLEANUP(pthread_mutex_destroy(&mutex));
 }