Reinitialize thread0's stack base after enabling XSAVE.

Otherwise the initial call to set_top_of_stack(), which occurs before
fpuinit() sets the correct value for cpu_max_ext_state_size, leaves the
stack base at an incorrect location. Then, when the full area is
zeroed, we end up erroneously zeroing part of the following page.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24754