Title: One line description
ppc64 build is not explicitly supported for wee8 gen_rule cmd file.

Description:
The ppc64le build has been failing since 319a9a6 was introduced. The gen_rule cmd does not include the instructions inside of https://github.com/envoyproxy/envoy/blob/master/bazel/external/wee8.genrule_cmd#L6 so it errors out.

I am working on a fix to insert the correct arch and flags to gn and also adding gn to the ppc build container.

Repro steps:
https://powerci.osuosl.org/job/build-envoy-static-master/2775/console