This commit introduces a trick for having optional arguments in
macros. This enables the `revng_abort` and `revng_unreachable` macros to
have 0 or 1 argument, and the `revng_assert` and `revng_check` macros to
have 1 or 2 arguments.

Note that, by employing a macro trick (instead of C++ default
arguments), we keep the header compatible with C files.