Adding section descriptions, for ELF: don't record sections satisfying

   size == 0
   ||
   kind `notElem` [SECTIONKIND_CODE_OR_RODATA, SECTIONKIND_RWDATA]

The latter condition is really an optimisation based on knowledge of
what queries will be made of the table.  Still, seems to work, and
reduces the number of sections in the list by about a factor of 3,
so hopefully will improve GC performance in GHCi.

If you get wierd GC problems in GHCi ... this may be to blame.