fix amd64 boot problems

the changes implemented in 1921a655 made the kernel able to boot images
of arbitrary size, without the user having to fine tune initial
bootstrap pmap parameters. this meant getting rid of fixed definitions
and calculating NKL2_KIMG_ENTRIES (the number of L2 page table entries
allocated for the kernel image) as well as TABLESIZE (the size of the
kernel's initial page tables) on the fly.

in order to avoid having to calculate TABLESIZE repeatedly, the code in
1921a655 calculated the value once and stored it on the stack, which at
this point is contained within the kernel image (tmpstk) and backed by
the initially established identity mapping.

we then proceed with the kernel's initialisation and, once in long mode,
dispose of the identity mapping. there is, however, one final reference
to TABLESIZE in the stack, which now lies in unmapped memory! note that
there are no TLB flushes inbetween, so albeit involving unmapped memory,
theoretically this access should work but, as demonstrated by QEMU when
configured with specific settings (i don't have the details) sometimes
it does not. in any case, this is a grey area -- and definitely
unintended behaviour! -- that can be easily circumvented by just
calculating TABLESIZE again, and this is what this diff does.

LGTM (like greek to me?) drahn@ patrick@