cmd: fix three printf problems reported by newest vet

Change-Id: Id70985d217c940eb022dbc95bfaa20373672512c
Reviewed-on: https://go-review.googlesource.com/c/go/+/712220
Auto-Submit: Alan Donovan <adonovan@google.com>
TryBot-Bypass: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/go/internal/modload/vendor.go

@@ -188,7 +188,7 @@ func checkVendorConsistency(loaderstate *State, indexes []*modFileIndex, modFile
 					// However, we can at least detect a version mismatch if packages were
 					// vendored from a non-matching version.
 					if vv, ok := vendorVersion[r.Mod.Path]; ok && vv != r.Mod.Version {
-						vendErrorf(r.Mod, fmt.Sprintf("is explicitly required in go.mod, but vendor/modules.txt indicates %s@%s", r.Mod.Path, vv))
+						vendErrorf(r.Mod, "is explicitly required in go.mod, but vendor/modules.txt indicates %s@%s", r.Mod.Path, vv)
 					}
 				} else {
 					vendErrorf(r.Mod, "is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt")

src/cmd/internal/obj/loong64/asm.go

@@ -4438,7 +4438,7 @@ func (c *ctxt0) specialFpMovInst(a obj.As, fclass int, tclass int) uint32 {
 		}
 	}
 
-	c.ctxt.Diag("bad class combination: %s %d,%s\n", a, fclass, tclass)
+	c.ctxt.Diag("bad class combination: %s %d,%d\n", a, fclass, tclass)
 
 	return 0
 }

src/cmd/internal/obj/riscv/obj.go

@@ -2991,7 +2991,7 @@ func (ins *instruction) length() int {
 func (ins *instruction) validate(ctxt *obj.Link) {
 	enc, err := encodingForAs(ins.as)
 	if err != nil {
-		ctxt.Diag(err.Error())
+		ctxt.Diag("%v", err)
 		return
 	}
 	enc.validate(ctxt, ins)