Add CI config to test on RBE. (#1638)

* Add CI config to test on RBE.

* Disable BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN.

This is set by default in the rbe_ubuntu1604 platform, but tests in this
repo need this to be disabled.

* Skip tests that are not RBE compatible.

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 68ce641..ec5f141 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -1,5 +1,6 @@
 workspace(name = "io_bazel_rules_go")
 
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 load("@io_bazel_rules_go//go:def.bzl", "go_rules_dependencies", "go_register_toolchains")
 load("@io_bazel_rules_go//proto:def.bzl", "proto_register_toolchains")
 
@@ -39,3 +40,12 @@
     name = "llvm_toolchain",
     llvm_version = "6.0.0",
 )
+
+http_archive(
+    name = "bazel_toolchains",
+    sha256 = "7938215442cc96b1a16809f4643db14af7d91c3ef1e98d39593bb46862204537",
+    strip_prefix = "bazel-toolchains-809c13c3a96966f63c7a43c586cb4e72589184b6",
+    urls = [
+        "https://github.com/bazelbuild/bazel-toolchains/archive/809c13c3a96966f63c7a43c586cb4e72589184b6.tar.gz",
+    ],
+)