Add a not-fully-functional C++ config for darwin (#228)

bazel/cc_toolchains/clang_toolchain.BUILD

@@ -20,10 +20,26 @@ filegroup(
 cc_toolchain_suite(
     name = "bazel_cc_toolchain",
     toolchains = {
+        "darwin": ":cc-compiler-darwin",
         "k8": ":cc-compiler-k8",
     },
 )
 
+cc_toolchain(
+    name = "cc-compiler-darwin",
+    all_files = ":empty",
+    ar_files = ":empty",
+    as_files = ":empty",
+    compiler_files = ":empty",
+    dwp_files = ":empty",
+    linker_files = ":empty",
+    objcopy_files = ":empty",
+    strip_files = ":empty",
+    supports_param_files = 1,
+    toolchain_config = ":local",
+    toolchain_identifier = "local",
+)
+
 cc_toolchain(
     name = "cc-compiler-k8",
     all_files = ":empty",