When linking mostly-static Linux binaries, link libstdc++.a explicitly.

This allows libstdc++ to be statically linked, which is normally only
possible when invoking GCC as `g++` with the `-static-libstdc++` flag.

Fixes bazelbuild#2840

See envoyproxy/envoy#415 for additional
background and context.

tools/cpp/CROSSTOOL

@@ -111,9 +111,23 @@ toolchain {
   tool_path { name: "dwp" path: "/usr/bin/dwp" }
   tool_path { name: "gcc" path: "/usr/bin/gcc" }
   cxx_flag: "-std=c++0x"
-  linker_flag: "-lstdc++"
   linker_flag: "-B/usr/bin/"
 
+  # `-static-libstdc++` is only supported when invoking GCC as `g++`, and
+  # `-lstdc++` forces dynamic linking of libstdc++. To get desired
+  # mostly-static behavior, invoke the link by explicitly naming a static
+  # library archive.
+  #
+  # https://github.com/bazelbuild/bazel/issues/2840
+  linking_mode_flags {
+    mode: DYNAMIC
+    linker_flag: "-lstdc++"
+  }
+  linking_mode_flags {
+    mode: MOSTLY_STATIC
+    linker_flag: "-l:libstdc++.a"
+  }
+
   # TODO(bazel-team): In theory, the path here ought to exactly match the path
   # used by gcc. That works because bazel currently doesn't track files at
   # absolute locations and has no remote execution, yet. However, this will need