Adds the rcheevos library from RetroAchievements to the Dolphin Externals as a submodule. Change was verified to import correctly and build both via Visual Studio and via cmake ninja.
* moves dolphin-specific settings out of Base.props
* creates exports.props for externals, allowing to easily import
individual Externals
* corrects some cruft that accumulated and probably contributed
to msbuild overbuilding
Add a function to calculate the magic constants required to optimize
signed 32-bit division.
Since this optimization is not exclusive to any particular architecture,
JitCommon seemed like a good place to put this.
