Anyone experimenting with different THREAD_SIZE or PAGE_SIZE on i386
runs into magical mystery numbers in do_boot_cpu esp initialization.
Remove those and use the same stack top in startup_32 as thereafter.

Oh, and what's that phys_to_virt(8192)?  Goodness! it's actually the
trampoline_base we (hopefully) got from early alloc_bootmem_low_pages.
Yes, could do with a lot more cleanup, but I'll stick here for safety.