Follow CL 146020 and enable RemoveAll based on Unlinkat and Openat on
freebsd.

Since the layout of syscall.Stat_t changes in FreeBSD 12, Fstatat needs
a compatibility wrapper akin to Fstatat in x/sys/unix. See CL 138595 and
CL 136816 for details.

Updates #27029

Change-Id: I8851a5b7fa658eaa6e69a1693150b16d9a68f36a
Reviewed-on: https://go-review.googlesource.com/c/146597
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>