Make compatible with the current MinGW cross-compiler

The Docker image was just updated to use Arch Linux instead of Ubuntu. This PR
sets the compiler names to reflect how they are installed on the Arch Linux
Docker image. Fixes "x86_64-w64-mingw32-g++-posix is not a full path and was not found in the PATH." errors in CMake.

CMakeModules/MinGWCross.cmake

@@ -10,8 +10,8 @@ set(SDL2_PATH                       ${MINGW_PREFIX})
 set(MINGW_TOOL_PREFIX               ${CMAKE_SYSTEM_PROCESSOR}-w64-mingw32-)
 
 # Specify the cross compiler
-set(CMAKE_C_COMPILER            ${MINGW_TOOL_PREFIX}gcc-posix)
-set(CMAKE_CXX_COMPILER          ${MINGW_TOOL_PREFIX}g++-posix)
+set(CMAKE_C_COMPILER            ${MINGW_TOOL_PREFIX}gcc)
+set(CMAKE_CXX_COMPILER          ${MINGW_TOOL_PREFIX}g++)
 set(CMAKE_RC_COMPILER           ${MINGW_TOOL_PREFIX}windres)
 
 # Mingw tools