Fixes a problem when using the external linker on Solaris.  The Solaris
external linker still doesn't work due to issue #14957.

The problem is, for example, with `go test cmd/objdump`:

        objdump_test.go:71: go build fmthello.go: exit status 2
                # command-line-arguments
                /var/gcc/iant/go/pkg/tool/solaris_amd64/link: running gcc failed: exit status 1
                Undefined                       first referenced
                 symbol                             in file
                x_cgo_callers                       /tmp/go-link-355600608/go.o
                ld: fatal: symbol referencing errors
                collect2: error: ld returned 1 exit status

Change-Id: I54917cfd5c288ee77ea25c439489bd2c9124fe73
Reviewed-on: https://go-review.googlesource.com/21392
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>