cmd/internal/obj/sparc64: make optab a map instead of a slice
cmd/internal/obj/sparc64: make optab a map instead of a slice

That way we get lookup for free, we don't need to sort it, and we
don't need to binary search it.

Change-Id: I4121d422ef0dfa31ee77de333fca13f1a45a931b

4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go