Fix aux_info corruption in rtld direct execution mode.

After the aux vector is moved, it is necessary to re-digest aux_info so the
pointers are updated to the new locations.

This was causing thread creation to fail on powerpc64 when using direct
execution due to a nonsense value being read for aux_info[AT_STACKPROT].

Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D21656