Supertramp: '' . . . famous last words . . . '' (A & M SP-3732.) Supertramp's long-awaited follow-up to their ''Breakfast in America'' album is an LP full of pleasant, tuneful songs and somewhat routine lyrics. Spending time with Supertramp is like spending time with some mild-mannered old friends. They're warm and welcome -- but not particularly challenging. The music flows freely from song to song in a smooth stream of mellow rockers. As their first cut, ''Crazy'' offers ''Here's a little song to make you feel good . . . '' Yet, Supertramp's sheer artistry and ability to orchestrate such a brilliantly appealing set of songs puts their music above the schlock-rock some of their more mundane work might aspire to. A good example is ''Put on Your Old Brown Shoes,'' which does what Supertramp does best.