I broke it with the darwin getwd attrlist stuff (0583e9d36dd).
plan9 doesn't have syscall.ENOTSUP.

It's in api/go1.txt as a symbol always available (not context-specific):

pkg syscall, const ENOTSUP Errno

... but plan9 isn't considered by cmd/api, so it only looks
universally available.  Alternatively, we could add a fake ENOTSUP
to plan9, but they were making efforts earlier to clean their
syscall package, so I'd prefer not to dump more in it.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12509044