Anybody who reaches Nirvana is considered Enlightened / Arahat. The Buddha is Enlightened. An Arahat is a word for an Enlightened being. Hence Buddha is also an Arahat.
Since the Nirvana for any person is the namesame the attainment of Arahatship is the same, though on other aspects the Buddha isBuddha's faculties, powers, attainments, insights and knowledges are far superior due to his Paramis.