Update RBE platform

 - Use the docker container from gcr.io/bazel-untrusted/ubuntu1604_bazel_rbe
   which was built specifically for Bazel's tests on RBE.
 - Introduce a highcpu_machine constraint in order to map heavy tests to
 beefy RBE workers.
 - Add the highcpu_machine constraints to heavy test targets.

Progress towards #8033

Closes #8062.

PiperOrigin-RevId: 243829775

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 93465e2..5f03fb4 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -277,13 +277,13 @@
 )
 
 http_archive(
-    name = "bazel_toolchains",
-    sha256 = "4b1468b254a572dbe134cc1fd7c6eab1618a72acd339749ea343bd8f55c3b7eb",
-    strip_prefix = "bazel-toolchains-d665ccfa3e9c90fa789671bf4ef5f7c19c5715c4",
-    urls = [
-        "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/d665ccfa3e9c90fa789671bf4ef5f7c19c5715c4.tar.gz",
-        "https://github.com/bazelbuild/bazel-toolchains/archive/d665ccfa3e9c90fa789671bf4ef5f7c19c5715c4.tar.gz",
-    ],
+  name = "bazel_toolchains",
+  sha256 = "67335b3563d9b67dc2550b8f27cc689b64fadac491e69ce78763d9ba894cc5cc",
+  strip_prefix = "bazel-toolchains-cddc376d428ada2927ad359211c3e356bd9c9fbb",
+  urls = [
+    "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/cddc376d428ada2927ad359211c3e356bd9c9fbb.tar.gz",
+    "https://github.com/bazelbuild/bazel-toolchains/archive/cddc376d428ada2927ad359211c3e356bd9c9fbb.tar.gz",
+  ],
 )
 
 # Creates toolchain configuration for remote execution with BuildKite CI
@@ -461,3 +461,5 @@
 load("@io_bazel_rules_sass//:defs.bzl", "sass_repositories")
 
 sass_repositories()
+
+register_execution_platforms("//:default_host_platform"); # buildozer: disable=positional-args