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

Updates #27029

Change-Id: I0b0e92f4422fa960a13dcd3e9adb57cd23f09ed4
Reviewed-on: https://go-review.googlesource.com/c/145839
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>