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/pthread_getspecific.c

@@ -1,4 +1,5 @@
 #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);