(struct stab_link_includes_totals): Add field 'symb' that keeps the characters

(struct stab_link_includes_totals): Add field 'symb' that keeps the characters
in a B_INCL..B_EINCL range.
(_bfd_link_section_stabs): When computing the sum of the characters in a
B_INCL..B_EINCL range also keep a copy of those	characters.  Use this
information to distinguish between include sections that have the same sum
and the same length but which are nevertheless unique.