x/tools: fastwalk broken on darwin

[bradfitz]

It seems directory reading is still broken on Darwin:

https://build.golang.org/log/e557926884d68397b4c4decd157f5947a6d0dff0

Bunch of fastwalk-related failures.

[gopherbot]

Change https://golang.org/cl/171477 mentions this issue: syscall: store skip count in file descriptor offset