[SystemZ] Avoid __INTPTR_TYPE__ conversions in vecintrin.h

Some intrinsics in vecintrin.h are currently implemented by
performing address arithmetic in __INTPTR_TYPE__ and converting
the result to some pointer type.  While this works correctly,
it leads to suboptimal code generation since many optimizers
cannot trace the provenance of the resulting pointers.

Fixed by using "char *" pointer arithmetic instead.