Refactor truncate floor and ceil duplicated part

[tompng]

Three functions are the same except trivial code styling and rounding mode part:

VpActiveRound(c.real, a.real, VP_ROUND_DOWN, iLoc); /* 0: truncate */
VpActiveRound(c.real, a.real, VP_ROUND_FLOOR, iLoc);
VpActiveRound(c.real, a.real, VP_ROUND_CEIL, iLoc);