[Bug libstdc++/91371] std::bind and bind_front don't work with function with call convention

[roland at rschulz dot eu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91371

--- Comment #2 from Roland Schulz <roland at rschulz dot eu> ---
Would you recommend to fix this by adding the specializations for the
alternative calling conventions to std::is_function or by switching to the
libc++ approach?