Reworked libretro solution

I copied the project files from each folder to our solution (GPU/ Common/ Core/ zlib/ libkirk) and readded our customizations, then I removed the libretro configs, so now on the libretro solution RELEASE means LIBRETRO RELEASE.

Also, I copied optimizations from the PPSSPP solution.
Still not building for 32 bit or in debug configs, I need help with those... not sure why they are not working.  

Also we need this one too
https://github.com/libretro/ppsspp-native/pull/5

I didn't revert the vcxproj files of the other solutions since I didn't find a clean way to do that. Anyway we no longer need to keep those up-to-date we can now focus on our own project files.