Support thread-local variables

[edsko]

Recent versions of the C standard support _Thread_local (until C23) / thread_local (since C23) variables.

For comparison/inspiration we could take a look at the Rust integration.

[edsko]

Closing this, as I believe it's not relevant. Most thread-local variables are within function bodies, which don't matter at all for the purposes of hs-bindgen. It is legal to have the thread_local annotation on global variables, and we might need to worry about that when generating bindings for global variables (especially if we use pointers to do so); added a comment to #42 that we should be careful with this.