misc/cgo/testplugin: disable DWARF tests on darwin

For some reason on darwin the linker still can't add debug sections to
plugins. Executables importing "plugin" do have them, however.

Because of issue 25841, plugins on darwin would likely have bad debug
info anyway so, for now, this isn't a great loss.

This disables the check for debug sections in plugins for darwin only.

Updates #27502

Change-Id: Ib8f62dac1e485006b0c2b3ba04f86d733db5ee9a
Reviewed-on: https://go-review.googlesource.com/133435


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>