Register allocation problems with TLS unit test.

Hello, Lang

The resulting code segfaults on execution. I haven't figured out
exactly why yet. Will post more details when I figure them out.

As far as I remember, it was a bug in binutils, which is workaround'ed
in gcc, but triggered with LLVM-generated code. I don't remember much
details, but I believe there was a binutils PR for this.