winpthreads: For DLL version use DllMain for calling TLS hook

Callbacks defined via PIMAGE_TLS_CALLBACK are not called on Win9x systems,
and also they are not called on all pre-Vista systems when DLL library is
loaded at runtime by LoadLibrary() call.

Co-authored-by: LIU Hao <lh_mouse@126.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>