BUILD: lua: silence some compiler warnings after WILL_LJMP

BUILD: lua: silence some compiler warnings after WILL_LJMP

These ones are on error paths that are properly handled by luaL_error()
which does a longjmp() but the compiler cannot know it. By adding an
__unreachable() statement in WILL_LJMP(), there is no ambiguity anymore.

This may be backported to 1.8 but these previous patches are needed first :
  - BUILD: compiler: add a new statement "__unreachable()"
  - MINOR: lua: all functions calling lua_yieldk() may return
  - BUILD: lua: silence some compiler warnings about potential null derefs (#2)