Upstream ps vita support from libretro

[phcoder]

[Screwtapello]

Is this based on the official Vita SDK, or a cleanly reverse-engineered homebrew devkit? If it's based on the official SDK, does this comply with whatever the NDA rules are? I'd hate to bring Sony's lawyers down on my head, or on the heads of people who included libco in some larger program.

[Screwtapello]

Also, libco.h declares a co_derive() function that seems to be missing here. Judging by the Windows Fibers implementation, it should be OK to just write a stub implementation if this particular operation doesn't make sense for this backend.