On some builders TestStatx occassionally fails when comparing atime
between Statx and Stat/Lstat.

--- FAIL: TestStatx (0.00s)
	syscall_linux_test.go:365: Statx: returned stat atime does not match Lstat

Fix it by dropping the atime comparison, it's enough to verify Statx
working correctly by comparing ctime and mtime. Also, not all
filesystems support atime.

Change-Id: I7da68bd20b9b21274c4993aa2c4241395d2c933a
Reviewed-on: https://go-review.googlesource.com/114616
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>