coredump: show supported coredump modes

Allow userspace to discover what coredump modes are supported.

Link: https://lore.kernel.org/20250516-work-coredump-socket-v8-6-664f3caf2516@kernel.org


Acked-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>