Add new __unreachable() builtin

This is one of the few post gcc4.2 builtins that has been implemented by
clang:

__builtin_unreachable is used to indicate that a specific point in the
program cannot be reached, even if the compiler might otherwise think it
can. This is useful to improve optimization and eliminates certain
warnings.

Hinted by: NetBSD
Differential Revision: https://reviews.freebsd.org/D2536