The precedent is set by the Redcap monster. It is a small Fey creature that has a special ability that lets it wield medium weapons "without penalty". Its default equipment is listed as a Medium Scythe. Now, a Scythe is a 2-h weapon when appropriately sized so a small creature would normally not be able to wield a medium 2-h weapon because the "effort to wield" step-up would take it above 2-h. However, given that the Redcap is written to wield such a weapon, we must conclude that "without penalty" applies not only to the -2 penalty to attack rolls but also counts the "effort step-up" as a penalty. This makes it distinctly different from something like the Titan Mauler who's ability specifically calls out "penalty to attack rolls for inappropriately sized weapons". Hence, since the Half-Giant's Powerful Build ability uses the same "without penalty" terminology, a Medium Half-Giant could wield a Large Longsword as if he were a Large creature. However, it also specifically calls out "weapons designed for a creature one size larger". So this ability only applies to the Large weapons (presuming a medium half-giant). So you couldn't say you're wielding a Medium Greatsword as a Large creature; a Greatsword is still a 2-h weapon for you, but a Large Longsword is still counted as a 1-h weapon, whereas for other medium creatures, it would "step-up" and count as a 2-h weapon and suffer a -2 penalty to attack. So your tactic would work only if you use a Large 1-h weapon; you'd hold the hammer in one hand (and not threaten with it) and make your full-attack with, say, a Large Longsword and threaten adjacent instead.
