By now all services in core where created, and registered in the generic
main routine. Although there exists already a x86-specific service (I/O ports)
there was no possibility to announce core-services for certain platforms only.
This commit introduces a hook function in the 'Platform' class, that enables
registration of platform-specific services. Moreover, the io-port service
is offered on x86 platforms only now.
Implement shared IRQs using 'Irq_proxy' class.
Nova: Added global worker 'Irq_thread' support in core and adapted Irq_session.
FOC: Adapted IRQ session code, x86 has shared IRQ support, ARM uses the old
model. Read and set 'mode' argument (from MADT) in 'Irq_session'.
OKL4: Use generic 'Irq_proxy'
Fixes issue #390
