fix reference to libc struct in static tls init code

libc is the macro, __libc is the internal symbol, but under some
configurations on old/broken compilers, the symbol might not actually
exist and the libc macro might instead use __libc_loc() to obtain
access to the object.

src/env/__init_tls.c

@@ -23,7 +23,7 @@ void *__copy_tls(unsigned char *mem)
 	td = (pthread_t)mem;
 	mem += sizeof(struct pthread);
 #else
-	mem += __libc.tls_size - sizeof(struct pthread);
+	mem += libc.tls_size - sizeof(struct pthread);
 	mem -= (uintptr_t)mem & (align-1);
 	td = (pthread_t)mem;
 	td->dtv = dtv;