x/sys/unix: NFDBITS not defined on aix

[tklauser]

From https://build.golang.org/log/2ab2cc7b53fb277213de58fb0017e2c4b362f0d1:

# golang.org/x/sys/unix [golang.org/x/sys/unix.test]
unix/fdset.go:11:14: undefined: NFDBITS
unix/fdset.go:16:14: undefined: NFDBITS
unix/fdset.go:21:21: undefined: NFDBITS

This is https://golang.org/cl/205397. I wasn't able to get access to the aix-ppc64 gomote to run mkall.sh/mkerrors.sh. Someone has a way to generate this?

@Helflym @ianlancetaylor @bradfitz

[gopherbot]

Change https://golang.org/cl/207657 mentions this issue: unix: regenerate zerrors for AIX