A 14-year-old arrested Saturday in the Oct. 6 stabbing death of a 39-year-old man on a Dorchester sidewalk will be charged as an adult, the Suffolk district attorney’s office said Sunday. The teen, whose identity will be released at his arraignment Monday in Dorchester District Court, will be charged with one count of murder. Cherby LaJoie was found with multiple stab wounds across the street from the Fields Corner T station, where he was pronounced dead. The death was the 44th homicide of the year in Boston. At the same time in 2011, 49 people had been killed, according to police.