syscall: let EPLAN9 and EWINDOWS implement errors.ErrUnsupported

syscall: let EPLAN9 and EWINDOWS implement errors.ErrUnsupported

As suggested by Bryan. This should fix the failing
TestIPConnSpecificMethods on plan9 after CL 476217 was submitted.

For #41198

Change-Id: I18e87b3aa7c9f7d48a1bd9c2819340acd1d2ca4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/476578
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>