Implement architecture-independent fallback in repositories.bzl

[brunoalr]

Archictecture-dependent binaries such as protoc limits portability to different targets (namely, I'm interested in Tensorflow ppc64le builds). I tried replacing x64 binaries with ppc64 in the cached dirs and managed to compile Tensorflow (see tensorflow/tensorflow#10306).

I could gladly send a pull request that does so, but I need some help to get a better understanding of bazel first, if that's possible.