build: ensure correct toolchains used for remote execution (#18803)

Currently the toolchains used for RBE are dependent on the
current platform. This breaks because there is no darwin toolchain.

We need to instruct Bazel to use the k8 linux toolchains when running
builds remotely.

(cherry picked from commit e3d2051)

Author: devversion

.bazelrc

@@ -76,6 +76,13 @@ build:remote --project_id=internal-200822
 # Needed due to: https://github.com/bazelbuild/bazel/issues/7254
 build:remote --define=EXECUTOR=remote
 
+# For remote execution, we use the `ubuntu16_04_clang` toolchain configurations. These only
+# support `k8` or `armeabi-v7a`. Since we run on remotely with `k8` containers, we need to
+# ensure that the proper CPU is configured so that the correct toolchain can be used.
+# https://github.com/bazelbuild/bazel-toolchains/blob/master/configs/ubuntu16_04_clang/10.0.0/bazel_2.1.0/cc/BUILD#L50
+build:remote --cpu=k8
+build:remote --host_cpu=k8
+
 # Setup the remote build execution servers.
 build:remote --remote_cache=remotebuildexecution.googleapis.com
 build:remote --remote_executor=remotebuildexecution.googleapis.com