The stat syscall is disallowed by Android's seccomp policy on
linux/amd64. Use the fstatat syscall with AT_FDCWD and zero flags to
achieve the same behavior.

CL 109835 did the same for package syscall.

Change-Id: Iaa7d0dbf17ade4803f59b4361973ce627bcb65b4
Reviewed-on: https://go-review.googlesource.com/110061
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>