Fix MinGW detection on x86
Fix MinGW detection on x86

Make detection of x64 using the gcc's target triplet identical
to the one used in cmake/OpenCVDetectCXXCompiler.cmake.
Otherwise, MinGW-w64 setups will always be treated as x64 since
they contain "w64" as vendor key.

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv