Although it has been nearly 20 years since he had his first hits, ''Gimme Some Lovin' '' and ''I'm a Man,'' with the Spencer Davis Group, Steve Winwood doesn't wear the predictable aura of a weather-beaten rock veteran. Indeed, the English pop-blues singer and multi-instrumentalist, who is a very young-looking 38 years old, is more popular today than in the late 1960's and early 70's, when he led the English folk-jazz-blues band Traffic. The spare textures and loosely blocked arrangements of his music today make only minor concessions to contemporary pop sounds as he continues to evoke an earlier, more romantic period in rock history.