altivec: fix macros

[Aldy Hernandez]

> bool is also required by the C++ standard, and the new ABI makes it the
> same size as a char.  Clobbering the compiler's idea of the bool type
> breaks ABI compatibility.  Please don't.

this is causing too much trouble.  unless someone can come up with
a good way of representing:

	vector bool short foo;

i'm going to remove the #definition of bool in altivec.h.  we're never
going to implement context sensitive keywords, so people will just
have to start using "vector signed short foo" instead.

any suggestions?