entry64: fix stack setup to survive kernel pagetable init
entry64: fix stack setup to survive kernel pagetable init

Use the high mapping instead of the bootstrap alias mapping.

When the kernel enables its own page tables, the 1:1 mapping
vaporizes and the kernel becomes unhappy if its stack vanishes
out from under it.

cpehle/xv6,maxymania/open-xv6,cpehle/xv6,maxymania/open-xv6