Instead of relying on manually passed down flags from CMake,
we now have ppsspp_config.h file to create the platform defines for us.
This improves support for multiplatform builds (such as iOS).
Opens up for having multiple JIT implementations available at runtime,
which could be use for experimenting with new JIT compiler types or for
unit testing one JIT on another architecture.
Very few of the newly virtual calls are on any sort of critical path so
hopefully there will not be a performance loss.
