If you don't set GOOS and you try to regenerate
the darwin files, it appears to succeed.
However, because libc was not selected,
it generates broken code.

Change-Id: I20fb4f51dcc539a03b6c6ed03ce39b78d5987d41
Reviewed-on: https://go-review.googlesource.com/c/161719
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>