libelf: Treat elf_memory as if using ELF_C_READ_MMAP

libelf: Treat elf_memory as if using ELF_C_READ_MMAP

An Elf handle created through elf_memory was treated as if opened with
ELF_C_READ. Which means libelf believed it had read the memory itself
and could simply write to it if it wanted (because it wasn't mmaped
directly on top of a file). This causes issues when that memory was
actually read-only. Work around this by pretending the memory was
actually read with ELF_C_READ_MMAP (so directly readable, but not
writable).

Add extra tests to elfgetzdata to check using elf_memory with
read-only memory works as expected.

	  * libelf/elf_memory.c (elf_memory): Call
	  __libelf_read_mmaped_file with ELF_C_READ_MMAP.
	  * tests/elfgetzdata.c (main): Add new "mem" option.
	  * tests/run-elfgetzdata.sh: Also run all tests with new
	  "mem" option.

https://sourceware.org/bugzilla/show_bug.cgi?id=31225

Reported-by: Derek Bruening <bruening@google.com>
Signed-off-by: Mark Wielaard <mark@klomp.org>