LLVM and TLS

Hello, Lauro.

I know there is a bug report about TLS (PR941). Is there someone
implementing TLS in LLVM and llvm-gcc? If not, I will implement this.

It's in my todo list. But I don't know, when I will have a chance to
start working. Adding TLS support seems to be tricky, since there are 4
different TLS "models" with different "assumptions".