[release-branch.go1.10] 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.

Updates #23672
Updates #23749

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 4a54ff21c25f1b5d7a4f73fd33214e1f8c83a9b9)
Reviewed-on: https://go-review.googlesource.com/94675


Reviewed-by: Andrew Bonventre <andybons@golang.org>