Get APs booting

OK, that's weird. Out of pure boredom, I though I just copy the ap_init code
over to 0xf0000 which should, according to the x86 memory map I thought I
had looked at, be freely available. (Maybe I should look at the e820 map,
too. My fault then.) However, qemu insisted not to let any copy to this area
happen. Now, I do the same things with 0x8000, overriding old BSP init code
for this simple testing purpose, and the things do work like a charm!

And, of course, the stosq had to be a movsq and %ecx must have been added 1
but even then it didn't work until I changed 0xf0000 to 0x8000...

Later today, I will clean that up (to ultimately understand what I did) and
more importantly I'll write the code more like it's in Pure64. The MINIX
way of preserving everything that is 'reserved' in the APIC registers is
fair enough but in assembly it just doesn't read very well :-)