cmd/gc: use appropriate verb to print array type length.

Fixes #4730.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8229043

src/cmd/gc/fmt.c

@@ -630,7 +630,7 @@ typefmt(Fmt *fp, Type *t)
 
 	case TARRAY:
 		if(t->bound >= 0)
-			return fmtprint(fp, "[%d]%T", (int)t->bound, t->type);
+			return fmtprint(fp, "[%lld]%T", t->bound, t->type);
 		if(t->bound == -100)
 			return fmtprint(fp, "[...]%T", t->type);
 		return fmtprint(fp, "[]%T", t->type);