libelf: Add elf32_getchdr, elf64_getchdr and gelf_getchdr.

Elf_Data of a compressed section has type ELF_T_CHDR. This type can be
xlated to the file or memory representation. This will make sure the Chdr
is in the correct endianess. The compressed data following the Chdr isn't
translated.

Signed-off-by: Mark Wielaard <mjw@redhat.com>