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 <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: tklauser

src/syscall/syscall_plan9.go

@@ -12,6 +12,7 @@
 package syscall
 
 import (
+	"errors"
 	"internal/oserror"
 	"runtime"
 	"unsafe"
@@ -43,6 +44,8 @@ func (e ErrorString) Is(target error) bool {
 	case oserror.ErrNotExist:
 		return checkErrMessageContent(e, "does not exist", "not found",
 			"has been removed", "no parent")
+	case errors.ErrUnsupported:
+		return checkErrMessageContent(e, "not supported")
 	}
 	return false
 }

src/syscall/syscall_windows.go

@@ -156,6 +156,8 @@ func (e Errno) Is(target error) bool {
 		return e == ERROR_FILE_NOT_FOUND ||
 			e == _ERROR_BAD_NETPATH ||
 			e == ERROR_PATH_NOT_FOUND
+	case errorspkg.ErrUnsupported:
+		return e == EWINDOWS
 	}
 	return false
 }