dlltool leading_underscore

This patch tidies dlltool code dealing with adding a leading
underscore to generated symbol names.  There should be no functional
change here, but there could be if we ever have a bfd target with
symbol_leading_char something other than '_' or 0.

	* dlltool.c (leading_underscore): Change from an int to a
        char*.  Update all uses.  If neither --leading-underscore or
        --no=leading-underscore is given, set leading_underscore to a
        string with first char returned by bfd_get_target_info as the
        target's symbol underscoring.