[k2] use cxx_toolchain_option (#1539)

Author: apolyakov

compiler/make/objs-to-k2-component-target.h

@@ -43,7 +43,8 @@ class Objs2K2ComponentTarget : public Target {
 public:
   std::string get_cmd() final {
     std::stringstream ss;
-    ss << settings->cxx.get() << additional_flags() << " -stdlib=libc++ -static-libstdc++ -shared -o " << target() << " ";
+    ss << settings->cxx.get() << " " << settings->cxx_toolchain_option.get() << " " << additional_flags() << " -stdlib=libc++ -static-libstdc++ -shared -o "
+       << target() << " ";
 
     for (size_t i = 0; i + 1 < deps.size(); ++i) {
       ss << deps[i]->get_name() << " ";