Fix crash when using wxThreadSpecificInfo from global object ctor.

When not using compiler TLS support (which is the default now), TLS variable
itself needs to be initialized and user-defined code in the global objects
ctor could be called before this happened, resulting in using uninitialized
CRITICAL_SECTION under Windows and a crash.

Fix this by wrapping global wxThreadSpecificInfo itself in an accessor
function ensuring that it is always initialized before use.

Notice that this required adding wxTLS_TYPE_REF() as wxTLS_TYPE() itself can't
be used for the function return value (__thread or similar can only be used on
the variables).

Closes #16009.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75949 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775