[builtins] Implement emulated TLS on Windows.

Summary:
LLVM JIT needs to be able to use emulated TLS on all platforms, and this provides a reference one can compile to enable emutls for Linux/Mac/Windows.

Reviewers: chh, howard.hinnant

Reviewed By: chh

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D30787

llvm-svn: 301350