powerpc: Enable 32-bit build

We have always built kvmtool as 64-bit on powerpc, but mainly just out
of habit. There's not AFAIK any reason we *can't* build 32-bit.

So fix up a few places where we were assuming 64-bit, and drop the
Makefile logic that forces 64-bit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Will Deacon <will.deacon@arm.com>