[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.
All BOOST_PLAT_WINDOWS_RUNTIME ifdefs need to be removed in favour of adjusting the code to support all platforms. I personally don't want to see a BOOST_PLAT_WINDOWS_RUNTIME unless there is really no possible choice. Here, for example, you can safely have a single start_thread_noexcept() with attributes parameter.