More warning cleanups

 - Add `#pragma warning` to dyncom and skyeye stuffs to ignore their warnings on MSVC
 - Add a `ToBool()` method to `BitField` to avoid manually/implicitly casting to bool
Heh, it's funny because earlier today I was addressing this myself, by adding a separate type for single-bit BitFields: https://github.com/neobrain/nihstro/blob/master/include/nihstro/bit_field.h#L201

You could port that one over to citra (and make work with other compilers than gcc) if you like. Otherwise, your current change is fine too, and I'll eventually add BitFlag myself.