Unbreak VKERNEL64 build.

Unbreak VKERNEL64 build.

After a05cac6 removal of MAKEBOOTDEV and B_ flags from sys/reboot.h
(that is not the place for such things) I broken the VKERNEL64 build.

This is a quick fix before someone with basic motoric functions
and a heart beat will take a better look (I lack the latter).

Not sure if disabling FFS_ROOT breaks anything, but it is better
than having build errors on vkernel/x86_64/autoconf.c

Also it gave me an oportunity to finally go through procedure
described in vkernel(7) to test it out for the first time!
So far no side effects on both X86_64_NON_GENERIC and VKERNEL64
except for no longer printed "no B_DEVMAGIC (bootdev=0)" in dmesg.

I should have guarded MAKEBOOTDEV and B_ flags with __i386__ in
sys/platform/pc64/include/bootinfo.h cause there is no platform/pc32
and I'm pretty sure thats a i386 thing only.