[release-branch.go1.12] os: don't treat RemoveAll("/x") as RemoveAll("x")

Updates #31468
Fixes #31474

Change-Id: I5c4e61631b8af35bfc14b0cb9bc77feec100e340
Reviewed-on: https://go-review.googlesource.com/c/go/+/172058


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 3ebd9523)
Reviewed-on: https://go-review.googlesource.com/c/go/+/172080