This extends #2929 to Darwin as well as Linux.
Running the example in https://github.com/golang/go/issues/37942
I see lots of:
```
dave@m1 sigurg % uname -ms
Darwin arm64
dave@m1 sigurg % go run main.go
received urgent I/O condition: 2021-05-21 16:03:03.482211 +0100 BST m=+0.014553751
received urgent I/O condition: 2021-05-21 16:03:03.507171 +0100 BST m=+0.039514459
```
Signed-off-by: David Scott <dave@recoil.org>