libelf: Treat elf_memory image as writable

There are use cases where the Elf image created by elf_memory is
manipulated, through the libelf interfaces, in place. This doesn't
work anymore since we changed elf_memory to assume the memory is
read-only in elfutils 0.191. commit cc44ac674 ('libelf: Treat
elf_memory as if using ELF_C_READ_MMAP').

The reason for that change was that if elf_memory was given a
read-only memory image then decompressing a section with elf_compress
would crash. Since it directly writes the updated Shdr size. If you do
want to use elf_compress on an Elf created by elf_memory you have make
sure the memory is writable. You can do this for example by using mmap
PROTE_WRITE and MAP_PRIVATE.

	* libelf/elf_memory.c (elf_memory): Call
	__libelf_read_mmaped_file with ELF_C_READ_MMAP_PRIVATE.
	* tests/elfgetzdata.c (main): Use mmap PROT_WRITE and MAP_PRIVATE.

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