[bazel] SkCMS RBE config for Linux.

After this CL lands, it will be possible to build and test SkCMS under Linux RBE by running the following commands:

    $ bazel build //... --config=linux-rbe
    $ bazel test //... --config=linux-rbe

The following also work:

    $ bazel test //... --config=linux-rbe --config=asan
    $ bazel test //... --config=linux-rbe --config=msan

This CL uses a custom Linux RBE toolchain container image based on a Google-provided[1] Ubuntu 18.04[2] base image. These images are scanned regularly for known vulnerabilities. It is our responsibility to rebuild our RBE toolchain container image to keep up with updates to the base image.

Please follow the instructions in //BAZEL.md to get the necessary RBE credentials.

Note to reviewers: The //bazel/toolchains/linux-bazel-4.2.1 directory was generated with the rbe_configs_gen tool from the https://github.com/bazelbuild/bazel-toolchains repository.


[1] https://cloud.google.com/artifact-management/docs/managed-base-images
[2] https://console.cloud.google.com/marketplace/product/google/ubuntu1804

Bug: skia:12400
Change-Id: If7588a00a0e56a12e431c7ced3125a1e317e4e05
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/437918
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Leandro Lovisolo <lovisolo@google.com>
18 files changed