CFR: a new __unreachable() builtin

Hello;

I am looking at the cdefs in other BSDs hoping to avoid adopting the
same definitions with incompatible names and I noticed NetBSD is using
a new __builtin_unreachable (void) function from gcc 4.6:

https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

Apparently it was interesting enough that clang implemented it too so
I created a code review differential for it.

https://reviews.freebsd.org/D2536

I don't want to add new C definitions unless they are going to be used
so feel free to comment on the convenience or not of having it.

Pedro.