EMERGING MARKETS-Brazil stocks down, America Movil weighs in Mexico

* Homebuilders slip after Brazil's MRV posts weak Q4 results * Mexico's America Movil falls on reform bill * Brazil Bovespa flat, Mexico IPC drops 1.72 pct By Asher Levine SAO PAULO, March 15 (Reuters) - Brazilian stocks fell on Friday after weak results from homebuilder MRV dampened the outlook for the country's construction sector, offsetting supportive economic growth data from the central bank. America Movil weighed on Mexico's bourse after a congressional committee pas