Make TLS keys actually take up space

If the TLS key is 0-sized, then the linux linker is apparently smart enough to
put everything at the same pointer. OSX on the other hand, will reserve some
space for all of them. To get around this, the TLS key now actuall consumes
space to ensure that it gets a unique pointer

src/test/compile-fail/core-tls-store-pointer.rs

@@ -12,6 +12,7 @@
 
 use std::local_data;
 
-static key: local_data::Key<@&int> = &[]; //~ ERROR only 'static is allowed
+static key: local_data::Key<@&int> = &local_data::Key;
+//~^ ERROR only 'static is allowed
 
 fn main() {}