prepare pthread_create.c for cherry-picking from master

this change is harmless and allows commit
a6adb2bcd8145353943377d6119c1d7a4242bae1 to apply without conflicts.

src/thread/pthread_create.c

@@ -110,6 +110,8 @@ static int start(void *p)
 /* pthread_key_create.c overrides this */
 static const size_t dummy = 0;
 weak_alias(dummy, __pthread_tsd_size);
+static void *const dummy_tsd[1] = { 0 };
+weak_alias(dummy_tsd, __pthread_tsd_main);
 
 static FILE *const dummy_file = 0;
 weak_alias(dummy_file, __stdin_used);