Use bind to redirect C++ crosstool configuration

This is a temporary workaround to enable the use
of a cc_configure() rules to auto-configure C++ toolchain.

--
MOS_MIGRATED_REVID=116140726
diff --git a/tools/cpp/BUILD b/tools/cpp/BUILD
index fd0a445..6e243e8 100644
--- a/tools/cpp/BUILD
+++ b/tools/cpp/BUILD
@@ -25,6 +25,12 @@
 # protobuf.
 filegroup(
     name = "toolchain",
+    srcs = ["//external:cc_toolchain"],
+)
+
+# Hardcoded toolchain, legacy behaviour.
+filegroup(
+    name = "default-toolchain",
     srcs = [
         ":cc-compiler-armeabi-v7a",
         ":cc-compiler-darwin",