Update third_party/llvm-7.0/scripts/update.py to pull in PPC64 LLVM
sources.

Bug: b/135175069
Change-Id: I2d5d61a18b822d3a10d958e3d20d314f62d51226
Signed-off-by: Colin Samples <colin.samples+git@gmail.com>
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32808Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>