The Darwin-only restriction was because we were late in the Go 1.7
cycle when the test was added.

In the process, I noticed Gettimeofday wasn't in the "unimplemented
midden heap" section of syscall_nacl.go, despite this line in the
original go1.txt:

pkg syscall, func Gettimeofday(*Timeval) error

So, add it, returning ENOSYS like the others.

Change-Id: Id7e02e857b753f8d079bee335c22368734e92254
Reviewed-on: https://go-review.googlesource.com/26772
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>