llvmpipe: add POWER8 portability file - u_pwr8.h

llvmpipe: add POWER8 portability file - u_pwr8.h

This file provides a portability layer that will make it easier to convert
SSE-based functions to VMX/VSX-based functions.

All the functions implemented in this file are prefixed using "vec_".
Therefore, when converting from SSE-based function, one needs to simply
replace the "_mm_" prefix of the SSE function being called to "vec_".

Having said that, not all functions could be converted as such, due to the
differences between the architectures. So, when doing such
conversion hurt the performance, I preferred to implement a more ad-hoc
solution. For example, converting the _mm_shuffle_epi32 needed to be done
using ad-hoc masks instead of a generic function.

All the functions in this file support both little-endian and big-endian
but currently the file is build only on POWER8 LE machine.

All of the functions are implemented using the Altivec/VMX intrinsics,
except one where I needed to use inline assembly (due to missing
intrinsic).

v2:
- Use vec_vgbbd instead of __builtin_vec_vgbbd
- Add an aligned load function
- Don't use typeof()
- Make file build only on POWER8 LE machine

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