This allows the removal of a fudge in data.go.

We have to defer the calls to adddynlib on non-Darwin until after we have
decided whether we are externally or internally linking.  The Macho/ELF
separation could do with some cleaning up, but: code freeze.

Fixing this once rather than per-arch is what inspired the previous CLs.

Change-Id: I0166f7078a045dc09827745479211247466c0c54
Reviewed-on: https://go-review.googlesource.com/10002
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>