Added default flags for all of the compilation modes.

toolchain/cc_toolchain_config.bzl

@@ -154,6 +154,21 @@ def _impl(ctx):
                   ),
               ],
           ),
+          flag_set(
+              actions = all_compile_actions,
+              flag_groups = [flag_group(flags = ["-DNDEBUG", "-O3"])],
+              with_features = [with_feature_set(features = ["opt"])],
+          ),
+          flag_set(
+              actions = all_compile_actions,
+              flag_groups = [flag_group(flags = ["-g"])],
+              with_features = [with_feature_set(features = ["dbg"])],
+          ),
+          flag_set(
+              actions = all_compile_actions,
+              flag_groups = [flag_group(flags = ["-O1"])],
+              with_features = [with_feature_set(features = ["fastbuild"])],
+          ),
       ],
   )