libsel4camkes: IRQ backend for global-connectors

This adds a way for calling registered IRQ notification handlers for
connectors that don't have their own threads. Instead the notifications
are delivered by calling camkes_handle_global_endpoint_irq() with a
provided badge corresponding to the received notifications. Connectors
are responsible for ensuring that the right symbols exist for linking
the badge value with the IRQ callback handler registered by earlier
calls to ps_irq_ops_t.

libsel4camkes: Update IRQ callback type

Following the change of the IRQ callback's type name in libplatsupport,
this commit updates the sources to reflect the change.

libsel4camkes: Move ELF section externs to sources

Instead of leaving the ELF section extern declarations in header files
where it could be redefined each time it's included, we move the
declarations to the .c files.

libsel4camkes: Add new IRQ interface

This interface slightly extends the IRQ interface in libplatsupport.