Remove libco

Created by: jdgleaver

This PR removes the core's dependency on libco. It is mostly derived from hiddenasbestos' puae fork (https://github.com/hiddenasbestos/libretro-uae/tree/gridc), although the implementation there was somewhat incomplete and required a little tweaking :)

It is difficult to benchmark this core, but from limited testing on my dev machine (using fast forward and monitoring the framerate) this PR seems to improve performance by 8-9%.

Perhaps more importantly, libco is a 'last resort' hack with a number of downsides (as explained by @realnc in PR #224) - removing it is in general a good thing.

@sonninnos I would really appreciate it if you could review this PR. The changes (and the parts I copied from hiddenasbestos) seem rational to me, and everything seems to work correctly - but I don't think this should be merged without the approval of our resident puae expert!