[release-branch.go1.9] cmd/go: permit pkg-config flags in any argument position
    
    Fixes #23875
    
    Change-Id: I503af71f44d11cd6b787fef100246b55735614a0
    Reviewed-on: https://go-review.googlesource.com/94896
    Run-TryBot: Ian Lance Taylor <iant@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    Reviewed-on: https://go-review.googlesource.com/103155
    Run-TryBot: Andrew Bonventre <andybons@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>