os: prevent RemoveAll to remove "." on Plan 9

CL 150497 enabled TestRemoveAllDot on "noat" systems.

However, this test is failing on Plan 9 because the rmdir
system call allows to remove "." on Plan 9.

This change prevents the "noat" implementation of RemoveAll to
remove ".", so it remains consistent with the "at" implementation.

Fixes #28903.

Change-Id: Ifc8fe36bdd8053a4e416f0590663c844c97ce72a
Reviewed-on: https://go-review.googlesource.com/c/150621


Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>