llvmpipe: Optimize lp_rast_triangle_32_3_16 for POWER8

llvmpipe: Optimize lp_rast_triangle_32_3_16 for POWER8

This patch converts the SSE-optimized lp_rast_triangle_32_3_16()
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
------------------------------------------------
openarena        16.35      16.7     2.14%
xonotic          4.707      4.97     5.57%

glmark2 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>