Set the return address for stack entry points to zero.

Stack unwinders treat zero as a stop condition. The value on the stack can
be non-zero because thread stacks may be arbitrary memory provided via
pthread_attr_setstack(3) or may be recycled from previous threads.

Reference:
https://lists.freebsd.org/pipermail/freebsd-current/2017-August/066855.html
https://lists.freebsd.org/pipermail/freebsd-current/2017-October/067254.html

Discussed with: kib
MFC after: 1 week