Port CL 120295 from golang.org/x/sys/unix to the syscall package.

The ustat syscall has been deprecated on Linux for a long time and the
upcoming glibc 2.28 will remove ustat.h and it can no longer be used to
to generate the Ustat_t wrapper type. Since Linux still provides the
syscall, let's not break this functionality and add a private copy of
struct ustat so Ustat_t can still be generated.

Fixes golang/go#25990

Change-Id: I0dab2ba1cc76fbd21553b499f9256fd9d59ca409
Reviewed-on: https://go-review.googlesource.com/120563
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>