cmd/link: check for reflect.Value.MethodByName explicitly

cmd/link: check for reflect.Value.MethodByName explicitly

Currently we only check for reflect.Value.Method. And
reflect.Value.MethodByName is covered since it calls
reflect.Value.Method internally. But it is brittle to rely on
implementation detail of the reflect package. Check for
MethodByName explicitly.

Change-Id: Ifa8920e997524003dade03abc4fb3c4e64723643
Reviewed-on: https://go-review.googlesource.com/c/go/+/228881
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>