This allows the internal implementation details of the TLS keys to be
changed without requiring the update of all the users. (Or, applying
changes that have to be applied for the keys to work correctly, e.g.
forcing LLVM to not merge these constants.)
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
