Previously core itself was the library containing the code to gather
common information (build info, CPU info, and OS info), however all of
this isn't core-dependent and can be moved to the common code and use
the common interfaces. We can then just call those functions from the
core instead.

This will allow replacing our CPU detection with Xbyak's which has
better detection facilities than ours. It also keeps more
architecture-dependent code in common instead of core.