runtime: Define BOOL to be the same as bool

As we define the ABI, we can just replace BOOL with bool everywhere,
including in ObjFW itself. For the Apple platforms where BOOL and bool
are different, this is not a problem as BOOL and bool are passed and
returned the same way in the ABI.

This still defines BOOL to bool for compatibility, except on AmigaOS and
Wii, which both have its own BOOL type.