[release-branch.go1.9] cmd/compile: permit go:cgo_import_dynamic anywhere
    
    It's used on Solaris to import symbols from shared libraries, e.g., in
    golang.org/x/sys/unix and golang.org/x/net/internal/socket.
    We could use a different directive but that would require build tags
    in all the places that use it.
    
    Fixes golang/go#23939
    
    Change-Id: I47fcf72a6d2862e304204705979c2056c2f78ec5
    Reviewed-on: https://go-review.googlesource.com/94018
    Run-TryBot: Andrew Bonventre <andybons@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    (cherry picked from commit 41d3d153eb76f000e9f6e9edf68ed441d15081cb)
    Reviewed-on: https://go-review.googlesource.com/110077
    Reviewed-by: Ian Lance Taylor <iant@golang.org>