libelf: Fix memory leak in elf_compress for mmapped ELF files.

libelf: Fix memory leak in elf_compress for mmapped ELF files.

The testcase added to run-strip-reloc.sh for strip-compressed.o showed
a memory leak when ran under valgrind (configure --enable-valgrind).

For a mmapped ELF file when existing section data was compressed
elf_end would fail to release the new compressed data buffer assigned
to rawdata_base. For non-mapped files rawdata_base is always freed.
For decompressed data rawdata_base is released together with zdata_base.

Use the Elf_Scn flag ELF_T_MALLOCED to track whether rawdata_base
points to malloced memory and free it in elf_end even for mmapped
ELF files.

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