Move `trace` flag from `compiler.Backend` interface to implementation struct field

[emil14]

Only golang.Backend, native.Backend and wasm.Backend actually support trace flag and passing it to ir.Backend or dot.Backend has no effect. The same will be true for any backend that doesn't actually produce executable (on one way or another). That's why trace bool flag must be moved from compiler.Backend.Emit method and should become part of the implementation