llvmpipe: Optimize BUILD_MASK(_LINEAR) for POWER8

llvmpipe: Optimize BUILD_MASK(_LINEAR) for POWER8

This patch converts the SSE-optimized build_mask_32() and
build_mask_linear_32() to VMX/VSX.

I measured the results on POWER8 machine with 32 cores at 3.4GHz and
16GB of RAM.

                      FPS/Score
  Name            Before     After    Delta
------------------------------------------------
glmark2 (score)   139.8      142.7    2.07%

openarena and xonotic didn't show a significant (more than 1%)
difference.

v2: Make sure code is build only on POWER8 LE machine

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>