os: fix plan9 build
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