you can't. you can only access the square end buffer through Buffer—Help | ArcGIS Desktop
You only other options would be to construct perpendicular lines at the line ends (for instance) and split the polygon there, or construct offset parallels and join those to form polygons.
Thank you Dan. I really didn't expect that it's impossible to do that directly as I thought maybe some arcmap option can do. So have to use the indirect way as your suggestion.