syscall: let ENOSYS, ENOTSUP and EOPNOTSUPP implement errors.ErrUnsupported

syscall: let ENOSYS, ENOTSUP and EOPNOTSUPP implement errors.ErrUnsupported

As suggested by Bryan, also update (Errno).Is on windows to include the
missing oserror cases that are covered on other platforms.

Quoting Bryan:
> Windows syscalls don't actually return those errors, but the dummy Errno
> constants defined on Windows should still have the same meaning as on
> Unix.

Updates #41198

Change-Id: I15441abde4a7ebaa3c6518262c052530cd2add4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/476875
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>