libelf: Replace list of elf_getdata_rawchunk results with a tree

libelf: Replace list of elf_getdata_rawchunk results with a tree

elf_getdata_rawchunks did a linear search to see if a chunk was
already fetched. Replace this list with a binary search tree to make
lookup faster when a lot of Elf_Data_Chunk were created.

       * libelf/libelfP.h (Elf_Data_Chunk): Remove next field.
       (struct Elf): Change the rawchunks type from Elf_Data_Chunk *
       to void *.
       * elf_getdata_rawchunk.c (chunk_compare): New static function.
       (elf_getdata_rawchunk): Use tsearch instead of a manual linked
       list.
       * elf_end.c (free_chunk): New static function.
       (elf_end): Call tdestroy instead of walking linked list.

Signed-off-by: Mark Wielaard <mark@klomp.org>