os: make RemoveAll("") fail silently on unix

CL 146020 changed the behavior of RemoveAll("") on unix systems using
the *at functions to return syscall.EINVAL instead of nil. Adjust the
*at implementation to retain this behavior as is the case on the *noat
systems.

Additionally, also make sure RemoveAll("") on systems not using the "at
functions (e.g. nacl and js/wasm) follow the same behavior (which wasn't
the case previously).

Fixes #28830

Change-Id: I8383c1423fefe871d18ff49134a1d23077ec6867
Reviewed-on: https://go-review.googlesource.com/c/150158


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>