cmd/link: detect -X setting non-string variable

Fixes #9621.

Change-Id: Ib9c6001378364af899f57fd4b89fb23af2042923
Reviewed-on: https://go-review.googlesource.com/11694
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
diff --git a/test/linkx.go b/test/linkx.go
index 40ec1b9..ac20334 100644
--- a/test/linkx.go
+++ b/test/linkx.go
@@ -14,6 +14,9 @@
 var tbd string
 var overwrite string = "dibs"
 
+var b bool
+var x int
+
 func main() {
 	fmt.Println(tbd)
 	fmt.Println(overwrite)