add C11 thread functions operating on tss_t and once_flag

These all have POSIX equivalents, but aside from tss_get, they all
have minor changes to the signature or return value and thus need to
exist as separate functions.

src/thread/call_once.c

+#include <threads.h>
+
+int __pthread_once(once_flag *, void (*)(void));
+
+void call_once(once_flag *flag, void (*func)(void))
+{
+	__pthread_once(flag, func);
+}

src/thread/pthread_getspecific.c

 #include "pthread_impl.h"
+#include <threads.h>
 
 static void *__pthread_getspecific(pthread_key_t k)
 {
@@ -7,3 +8,4 @@ static void *__pthread_getspecific(pthread_key_t k)
 }
 
 weak_alias(__pthread_getspecific, pthread_getspecific);
+weak_alias(__pthread_getspecific, tss_get);

src/thread/tss_create.c

+#include <threads.h>
+
+int __pthread_key_create(tss_t *, void (*)(void *));
+
+int tss_create(tss_t *tss, tss_dtor_t dtor)
+{
+	/* Different error returns are possible. C glues them together into
+	 * just failure notification. Can't be optimized to a tail call,
+	 * unless thrd_error equals EAGAIN. */
+	return __pthread_key_create(tss, dtor) ? thrd_error : thrd_success;
+}

src/thread/tss_delete.c

+#include <threads.h>
+
+int __pthread_key_delete(tss_t k);
+
+void tss_delete(tss_t key)
+{
+	__pthread_key_delete(key);
+}

src/thread/tss_set.c

+#include "pthread_impl.h"
+#include <threads.h>
+
+int tss_set(tss_t k, void *x)
+{
+	struct pthread *self = __pthread_self();
+	/* Avoid unnecessary COW */
+	if (self->tsd[k] != x) {
+		self->tsd[k] = x;
+		self->tsd_used = 1;
+	}
+	return thrd_success;
+}