Define local instances of FALLTHROUGH and UNREACHABLE

FALLTHOUGH is a copy of how it is defined in <isc/util.h>

UNREACHABLE follows the model used in MacOS /usr/include/c++/v1/cstdlib to determine if __builtin_ureachable is available

(cherry picked from commit 6d68a229)

Closes #3306 (closed)

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