cmd/compile: declare with type for fmtMode constant

Like FmtFlag constant in fmt.go

Change-Id: I351bcb27095549cf19db531f532ea72d5c682610
Reviewed-on: https://go-review.googlesource.com/c/go/+/209497
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/fmt.go

@@ -94,7 +94,7 @@ func fmtFlag(s fmt.State, verb rune) FmtFlag {
 
 // *types.Sym, *types.Type, and *Node types use the flags below to set the format mode
 const (
-	FErr = iota
+	FErr fmtMode = iota
 	FDbg
 	FTypeId
 	FTypeIdName // same as FTypeId, but use package name instead of prefix

src/cmd/compile/internal/gc/main.go

@@ -526,7 +526,7 @@ func Main(archInit func(*Arch)) {
 	}
 	types.FmtLeft = int(FmtLeft)
 	types.FmtUnsigned = int(FmtUnsigned)
-	types.FErr = FErr
+	types.FErr = int(FErr)
 	types.Ctxt = Ctxt
 
 	initUniverse()