Adds support for PPC64 LLVM build to CMake and gn

Bug: b/135175069
Signed-off-by: Colin Samples <colin.samples+git@gmail.com>
Change-Id: I3123a6e1457d3a5b2528516dadc019617afa4988
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32810
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>