docs/llvmpipe: document ppc64le as alternative architecture to x86.

Power8, Power8NV, and Power9 are supported on an equal footing
with X86.

Cc: "17.2" "17.3" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Ben Crocker <bcrocker@redhat.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

[Eric: changed formatting, reworded a bit (with Ben's ack)]
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
(cherry picked from commit 060eb314)