avoid use of '__builtin_unreachable' in 'UNREACHABLE' macro

avoid use of '__builtin_unreachable' in 'UNREACHABLE' macro

There was some discussion on !2503¹ around the effect of `__builtin_unreachable`
and whether we are really confident applying such a strong compiler hint to
complex control flow logic that is, in some cases, only partially understood.
This change softens the effect of the `UNREACHABLE` macro to a reliable program
abort rather than undefined behavior if mistakenly tagged unreachable code is
reached. The result is slightly less efficient but safer code.

¹ https://gitlab.com/graphviz/graphviz/-/merge_requests/2503#note_866261546