iOS patch no 3: enable/disable terminfo more precisely by os
When cross compiling to iOS, HostOS = darwin and TargetOS = ios. darwin has termios but ios doesn't. So we need termios built in stage 0 but not stage 1, but the makefiles don't currently allow for that.
This patch makes it so termios is selected by the right OS for the right stage. I've refactored how Windows is handled here, so e.g. the termios selection should work correctly for both native windows and a Linux -> mingw32 cross compile (though I haven't tested it). Linux -> Windows mirrors darwin -> ios, in that it's termios -> no termios.