Spectres should be supportable by every support gem

Spectres have similar skills to the player, and on paper these skills could be supported by the same gems the player would use. However, the Spectre skill gem does not allow them to be supported by many support gems. This ends up heavily restricting the number of usable spectres.

Right now, there are only a handful of units whose abilities can be fully supported, and those end up being the best-in-slot units because of this.