gallivm: Altivec vector add/sub intrisics

gallivm: Altivec vector add/sub intrisics

This patch add correct vector addition and substraction intrisics when
using Altivec with PPC. Current code uses default path and LLVM backend
ends up issuing carry-out arithmetic instruction while it is expected
saturated ones.

It also includes a fix for PowerPC where char are unsigned by default,
resulting in bogus values for vector shifting.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>