Implement static_link_cpp_runtimes feature

This patch implements the `static_link_cpp_runtimes` feature
in `unix_cc_toolchain_config`.  The feature is documented as
a well-known feature[1] and was added to the toolchain[2] but
not implemented.

The patch follows the solution proposed on GitHub.[3]

For the feature to work as expected, users have to remove
the linker flag `"-lstdc++"` from `link_libs` (or `link_flags`)
from the call to `unix_cc_toolchain_config`.

[1] https://bazel.build/docs/cc-toolchain-config-reference#wellknown-features
[2] bazelbuild#17391
[3] bazelbuild#14342

tools/cpp/unix_cc_toolchain_config.bzl

@@ -269,6 +269,17 @@ def _impl(ctx):
     static_link_cpp_runtimes_feature = feature(
         name = "static_link_cpp_runtimes",
         enabled = False,
+        flag_sets = [
+            flag_set(
+                actions = [
+                    ACTION_NAMES.cpp_link_executable,
+                    ACTION_NAMES.cpp_link_dynamic_library,
+                    ACTION_NAMES.lto_index_for_executable,
+                    ACTION_NAMES.lto_index_for_dynamic_library,
+                ],
+                flag_groups = [flag_group(flags = ["-static-libstdc++"])],
+            ),
+        ],
     )
 
     default_compile_flags_feature = feature(
@@ -1059,6 +1070,15 @@ def _impl(ctx):
                     ),
                 ],
             ),
+            flag_set(
+                actions = all_link_actions + lto_index_actions,
+                flag_groups = [flag_group(flags = ["-lstdc++"])],
+                with_features = [
+                    with_feature_set(
+                        not_features = ["static_link_cpp_runtimes"],
+                    ),
+                ],
+            ),
         ],
     )