BUILD: compiler: add a new statement "__unreachable()"

BUILD: compiler: add a new statement "__unreachable()"

This statement is used as a hint for the compiler so that it knows that
the location where it's placed cannot be reached. It will mostly be used
after longjmp() or equivalent statements that deal with error processing
and that the compiler doesn't know will not return on certain conditions,
so that it doesn't complain about null dereferences on error paths.