implement pthread_rwlockattr_* (essentially no-ops)

src/thread/pthread_rwlockattr_destroy.c

+#include "pthread_impl.h"
+
+int pthread_rwlockattr_destroy(pthread_rwlockattr_t *a)
+{
+	return 0;
+}

src/thread/pthread_rwlockattr_getpshared.c

+#include "pthread_impl.h"
+
+int pthread_rwlockattr_getpshared(const pthread_rwlockattr_t *a, int *pshared)
+{
+	*pshared = *(int *)a;
+	return 0;
+}

src/thread/pthread_rwlockattr_init.c

+#include "pthread_impl.h"
+
+int pthread_rwlockattr_init(pthread_rwlockattr_t *a)
+{
+	memset(a, 0, sizeof *a);
+	return 0;
+}

src/thread/pthread_rwlockattr_setpshared.c

+#include "pthread_impl.h"
+
+int pthread_rwlockattr_setpshared(pthread_rwlockattr_t *a, int pshared)
+{
+	if (pshared > 1U) return EINVAL;
+	*(int *)a = pshared;
+	return 0;
+}