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.

cmake/OpenCVConfig.cmake

@@ -86,7 +86,7 @@ elseif(MINGW)
   execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpmachine
                   OUTPUT_VARIABLE CMAKE_OPENCV_GCC_TARGET_MACHINE
                   OUTPUT_STRIP_TRAILING_WHITESPACE)
-  if(CMAKE_OPENCV_GCC_TARGET_MACHINE MATCHES "64")
+  if(CMAKE_OPENCV_GCC_TARGET_MACHINE MATCHES "amd64|x86_64|AMD64")
     set(MINGW64 1)
     set(OpenCV_ARCH x64)
   else()