syscall: replace mksyscall_windows.pl with mksyscall_windows.go
Not many windows users have perl installed. They can just use
standard go tools instead. Also mkerrors_windows.sh script
removed - we don't add any new "unix" errors to windows
syscall package anymore.
Apparently I decided we won't be making changes to zerrors_windows.go.
I suppose it is fine to stay as is. So closing this issue.