Support illumos.

[jperkin]

You may prefer to change this completely to instead be !windows, I didn't do this as I don't know your reasoning for having a static OS list, but it would avoid having to add other OS manually in the future.

I cross-compiled this on macOS:

$ GOOS=illumos GOARCH=amd64 go test -c

and ran the resulting msgp.test on an illumos host:

$ ./msgp.test 
PASS

to verify.

Thanks!