For given program with 2 undefined relocations (main and undefined):

	package main
	func undefined()
	func defined() int {
		undefined()
		undefined()
		return 0
	}
	var x = defined()

"go tool link" produces these errors:

	main.defined: relocation target main.undefined not defined
	main.defined: relocation target main.undefined not defined
	runtime.main_main·f: relocation target main.main not defined
	main.defined: undefined: "main.undefined"
	main.defined: undefined: "main.undefined"
	runtime.main_main·f: undefined: "main.main"

After this CL is applied:

	main.defined: relocation target main.undefined not defined
	runtime.main_main·f: function main is undeclared in the main package

Fixes #10978
Improved error message for main proposed in #24809.

Change-Id: I4ba8547b1e143bbebeb4d6e29ea05d932124f037
Reviewed-on: https://go-review.googlesource.com/113955
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>