work around gcc-14.2 discomfiture about constinit shared_ptr. (#1565)

Author: ericniebler

include/exec/__detail/__system_context_default_impl.hpp

@@ -301,6 +301,10 @@ namespace exec::__system_context_default_impl {
   /// Keeps track of the backends for the system context interfaces.
   template <typename _Interface, typename _Impl>
   struct __instance_data {
+    // work around for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119652
+    constexpr __instance_data() noexcept // NOLINT(modernize-use-equals-default)
+    {}
+
     /// Gets the current instance; if there is no instance, uses the current factory to create one.
     auto __get_current_instance() -> std::shared_ptr<_Interface> {
       // If we have a valid instance, return it.