Implement optional frameskip feature.

On mobile hardware, this makes a big difference in playability on most ROMs.  E.g. on the Tegra3-based Nexus 7 (2012) and OUYA, games like DK64 and Diddy Kong Racing are not playable without this feature.

I understand that frameskip is blasphemy in some communities, so this feature is only enabled via build flag.  In other words, by default, this PR makes no change whatsoever to the compiled binary.

@paulscode