add support for the x86 APIC architecture

Tested with a local APIC in UP mode, and a single I/O APIC in qemu-kvm.
Whether or not the PIC-only mode works has not been tested.
exception: recognize kernel #GP from frame length

And not from which thread was scheduled into most recently.
apic: add disable switch

Add -DDISABLE_APIC to config.mk to activate it. Its presence breaks all
function-related interrupt tests, though, as IRQs 2..15 haven't got service
routines or IDT entries. This should be remedied with a generic handler that
pokes the PICs' service masks, much as the APIC ISR does now, and later (v1,
before SMP support) optimized with generated code per vector so that there's
no more poking through the interrupt controller's masks.
add L4X2::ExceptionHandler IDL

These should really be derived from L4Ka's equivalents, if there are any.
testbench: x86: add tests for exception and TF handling

The first works just fine. The second needs a patch to exception.c so that the
exception handler can change TF.
exception: pass the TF bit of EFLAGS via exception reply

It's only a system flag in the sense that it has to be loaded by supervisor
mode.