Add thread local storage API

[baylej]

I'd like to store per thread values, allegro does not expose such functionality from the underlying win32/pthread threading API.
Thanks.

[SiegeLord]

It's a good idea. This would simplify some of the addon code that really wants thread local storage, but couldn't use it before. One question to resolve is whether to stash this inside Allegro's TLS state somehow or expose the raw TLS API.