include: add a userland version of __assert_unreachable

[kevans]

Summary

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.

[collin.funk1_gmail.com]

C23 added unreachable () to <stddef.h>. Wouldn't that be preferable to the leading __ for userland?

[kevans]

In D48077#1129981, @collin.funk1_gmail.com wrote:

C23 added unreachable () to <stddef.h>. Wouldn't that be preferable to the leading __ for userland?

My can't assume C23 in system headers, so we'd end up shimming it out with an __unreachable in <sys/cdefs.h> anyways to make it usable