[libc][pthread] fix -Wmissing-field-initializers

Fixes:

    llvm-project/libc/test/integration/src/pthread/pthread_rwlock_test.cpp:59:29:
    warning: missing field '__preference' initializer
    [-Wmissing-field-initializers]
       59 |   pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
          |                             ^

Also, add a test that demonstrates the same issue for
PTHREAD_MUTEX_INITIALIZER, and fix that, too.

PTHREAD_ONCE_INIT does not have this issue and does have test coverage.

Author: nickdesaulniers

libc/include/llvm-libc-macros/pthread-macros.h

@@ -25,8 +25,8 @@
 #define PTHREAD_PROCESS_PRIVATE 0
 #define PTHREAD_PROCESS_SHARED 1
 
-#define PTHREAD_MUTEX_INITIALIZER {0}
-#define PTHREAD_RWLOCK_INITIALIZER {0}
+#define PTHREAD_MUTEX_INITIALIZER {}
+#define PTHREAD_RWLOCK_INITIALIZER {}
 
 // glibc extensions
 #define PTHREAD_STACK_MIN (1 << 14) // 16KB

libc/test/integration/src/pthread/pthread_mutex_test.cpp

@@ -186,6 +186,10 @@ void multiple_waiters() {
   LIBC_NAMESPACE::pthread_mutex_destroy(&counter_lock);
 }
 
+// Test the initializer
+[[gnu::unused]]
+static pthread_mutex_t test_initializer = PTHREAD_MUTEX_INITIALIZER;
+
 TEST_MAIN() {
   relay_counter();
   wait_and_step();