[winrt support] Enabling Boost.Thread to be used in the Windows Runtime.

This involves basically 3 changes:
1. Using __declspec(thread) instead of the Tls APIs.
2. Using Windows::System::Threading since Win32 Threading APIs aren't allowed.
3. Updating or replacing some banned APIs like WaitForSingleObject with WaitForSingleObjectEx.
You've just reminded me about a patch implementing GetTickCount64 on WinXP using atomics which I have forgotten about. It lives at https://github.com/boostorg/thread/pull/8 and I'll get it fixed up after I finish this review, hopefully we'll have it into HEAD soon so you can work from it.