cmd/link: in package .syso not working in Go 1.12

[minux]

Current go master (Go 1.12) broke the use of in-package syso files (broken by https://golang.org/cl/146297) with names shorter than 12 byte (because
the limitation on filename length, even though race detector relies on syso,
it is not broken by the CL.)

As a short reproduction, run the following shell script under $GOPATH/src:

#!/bin/bash

set -ex

mkdir issue
cd issue

cat <<EOF > ext.c
// +build ignore

int f(int x) { return x * 2; }
EOF
cat <<EOF > pkg.go
package issue

// extern int f(int);
import "C"

func init() {
	println(C.f(42))
}
EOF
cat <<EOF > main.go
// +build ignore

package main

import _ "issue"

func main() {}
EOF

cc -c -fPIC -o ext.syso ext.c
go run main.go

Because Go 1.12 cmd/link no longer passes the bundled syso to the external linker,
the final go run will fail to build the executable due to missing f symbol.

This is a Go 1.12 regression, so labelling it as release-broker.

[gopherbot]

Change https://golang.org/cl/154109 mentions this issue: cmd/link: fix in-package syso linking