llvmpipe: disable VSX in ppc due to LLVM PPC bug

llvmpipe: disable VSX in ppc due to LLVM PPC bug

This patch disables the use of VSX instructions, as they cause some
piglit tests to fail

For more details, see: https://llvm.org/bugs/show_bug.cgi?id=25503#c7

With this patch, ppc64le reaches parity with x86-64 as far as piglit test
suite is concerned.

v2:
- Added check that we have at least LLVM 3.4
- Added the LLVM bug URL as a comment in the code

v3:

- Only disable VSX if Altivec is supported, because if Altivec support
is missing, then VSX support doesn't exist anyway.

- Change original patch description.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Cc: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>