Add static TLS support to DR's Windows private loader

Split from #233. We are now hitting failures due to the lack of static TLS support: #4002 (closed), #3976

Note that pre-Vista static TLS did not exist in DLL's because the Windows loader itself did not support it. But we are now seeing it used for everyday things such as function-static class constructors with VS2017's C++ library: #4002 (closed). This is blocking us from moving to VS2017 (#2924 (closed)).