Fix huge performance problem in wxStdInputStream when using MSVC8/9.

Fix huge performance problem in wxStdInputStream when using MSVC8/9.

Our overridden xsgetn() method was never called when using these compilers
because they used their own, non-standard, _Xsgetn_s() instead. Because of
this the stream was always read character by character which was very
inefficient.

Fix the problem by overriding _Xsgetn_s() for these compilers and explicitly
forwarding it to xsgetn().

Closes #13926.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70515 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775