Account for the main process stack being one page below the highest
user address when ABI uses shared page.

Note that the change is no-op for correctness, since shared page does
not fault. The mapping for the shared page is installed at the
address space creation, the page is unmanaged and its pte/pv entry
cannot be reclaimed.

Submitted by: Oliver Pinter
Review: https://reviews.freebsd.org/D2954
MFC after: 1 week