Revert the vm_machdep.c part of r205072.
Revert the vm_machdep.c part of r205072.

This causes a panic in vm_thread_dispose() when it tries to add this kstack
to the kstack cache. This happens only when 'td_kstack' is not (PAGE_SIZE * 2)
bytes aligned and we have unmapped the page at that address in cpu_thread_alloc.

Pointed out by: nwhitehorn@

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