[libc++] Generalize the customizeable assertion handler

Instead of taking a fixed set of arguments, use variadics so that
we can pass arbitrary arguments to the handler. This is the first
step towards using the handler to handle other non-assertion-related
failures, like std::unreachable and an exception being thrown in
-fno-exceptions mode, which would improve user experience by including
additional information in crashes (right now, we call abort() without
additional information).

Differential Revision: https://reviews.llvm.org/D130507