syscall: use Android O friendly fstatat syscall to implement Stat on linux/amd64

syscall: use Android O friendly fstatat syscall to implement Stat on linux/amd64

The Android O seccomp policy disallows the stat syscall on amd64, see
https://android.googlesource.com/platform/bionic/+/android-4.2.2_r1.2/libc/SYSCALLS.TXT

Use the fstatat syscall with AT_FDCWD and zero flags instead to achieve
the same behavior.

Fixes #24403

Change-Id: I36fc9ec9bc938cd8e9de30f66c0eb9d2e24debf6
Reviewed-on: https://go-review.googlesource.com/100878
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>