Better integration of MinGW cross-compiling build process.

- Runtime and archive output directories change to mingw-Bin and mingw-Lib, respectively.
- Enable SCP_TO_TARGET build option for all cross-compiling build process (except Android).
- Enable LuaJIT support for MinGW cross-compiling build.
- Cache MinGW toolchain path and sysroot.
- Fix "object file is too big" issue when building Assimp library in debug build using MinGW.
- Reorder the build processes documentation section to match the Readme.txt file.