avoid local runs from accessing remote platforms

This should fix Bazel Basic Tests for Python (Local) and other
local CI runs that are now failing.
I also have a plan to fix Bazel RBE Opt C/C++.
Bazel RBE UBSAN C/C++ might need to wait for gRPC to upgrade to
using 0.23.0
diff --git a/WORKSPACE b/WORKSPACE
index fd77da4..9ed7776 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -9,12 +9,8 @@
 grpc_test_only_deps()
 
 register_execution_platforms(
-    "//third_party/toolchains:rbe_ubuntu1604",
-    "//third_party/toolchains:rbe_ubuntu1604_large",
-)
-
-register_toolchains(
-    "//third_party/toolchains:cc-toolchain-clang-x86_64-default",
+    "//third_party/toolchains:local",
+    "//third_party/toolchains:local_large",
 )
 
 http_archive(
diff --git a/third_party/toolchains/BUILD b/third_party/toolchains/BUILD
index 9a2eb32..36cfd57 100644
--- a/third_party/toolchains/BUILD
+++ b/third_party/toolchains/BUILD
@@ -78,12 +78,18 @@
     """,
 )
 
-toolchain(
-    name = "cc-toolchain-clang-x86_64-default",
-    exec_compatible_with = [
+platform(
+    name = "local",
+    parents = ["@bazel_tools//platforms:target_platform"],
+    constraint_values = [
+        "//third_party/toolchains/machine_size:standard",
     ],
-    target_compatible_with = [
+)
+
+platform(
+    name = "local_large",
+    parents = ["@bazel_tools//platforms:target_platform"],
+    constraint_values = [
+        "//third_party/toolchains/machine_size:large",
     ],
-    toolchain = "@rbe_default//cc:cc-compiler-k8",
-    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
 )
diff --git a/tools/remote_build/rbe_common.bazelrc b/tools/remote_build/rbe_common.bazelrc
index fb74af0..871f1d5 100644
--- a/tools/remote_build/rbe_common.bazelrc
+++ b/tools/remote_build/rbe_common.bazelrc
@@ -20,7 +20,7 @@
 startup --host_jvm_args=-Dbazel.DigestFunction=SHA256
 
 build --crosstool_top=@rbe_default//cc:toolchain
-build --extra_toolchains=//third_party/toolchains:cc-toolchain-clang-x86_64-default
+build --extra_toolchains=@rbe_default//config:cc-toolchain
 # Use custom execution platforms defined in third_party/toolchains
 build --extra_execution_platforms=//third_party/toolchains:rbe_ubuntu1604,//third_party/toolchains:rbe_ubuntu1604_large
 build --host_platform=//third_party/toolchains:rbe_ubuntu1604