include: add a userland version of __assert_unreachable

The kernel has had a version of this since
c79cee71363d ("kernel: provide panicky version of unreachable"), and
userland can benefit from the same. unreachable is largely
inadequate because it's *not* an assertion of any sort, so we're not
really alerted to a problem that we could've anticipated.

Reviewed by: emaste, imp, jhb, olce
Differential Revision: https://reviews.freebsd.org/D48077