[Suggestion] Remove IRONMAN requirement for "L" Signposts

Regardless of whether or not this privilege should exclusive to weird gimmick characters, Heavy Magician is the only character in the vanilla cast who has art for this, and the hardcode already has the ability to display fallback graphics for sprite2 sequences that don't exist on the current character.

All the code's already there, make SPR2_SIGL available to other characters. Creators may have ideas for other gimmicks that are worthy of "L" signposts, or they might just do it because they want to make more silly art. Either way, don't need to force creators to write Lua for this.