The symExport flag tells whether a symbol is in the export list
already or not (and it's also used to avoid being added to that
list). Exporting is based on that export list - no need to check
again.

Change-Id: I6056f97aa5c24a19376957da29199135c8da35f9
Reviewed-on: https://go-review.googlesource.com/39033Reviewed-by: Dave Cheney <dave@cheney.net>