libdwfl: Make dwfl_report_offline_memory work with ELF_C_READ_MMAP

libdwfl: Make dwfl_report_offline_memory work with ELF_C_READ_MMAP

elf_memory open mode recently changed from ELF_C_READ to
ELF_C_READ_MMAP. This broken dwfl_report_offline_memory that changes
mode to ELF_C_READ_MMAP_PRIVATE to be compatible with subsequent
elf_begin on embedded ELF files.

The proper implementation of dwfl_report_offline_memory doesn't change
open mode and subsequent elf_begin invocations simply use cmd from the
reference Elf*.

Add tests to exercise Elf* to trigger the bug caused by incorrect cmd
set to Elf*.

	  * libdwfl/offline.c (process_archive): Use archive->cmd
	  instead of hardcoded ELF_C_READ_MMAP_PRIVATE.
	  * libdwfl/open.c (libdw_open_elf): Use elf->cmd instead of
	  hardcoded ELF_C_READ_MMAP_PRIVATE.
	  (__libdw_open_elf_memory): Don't override (*elfp)->cmd.
	  * tests/Makefile.am (dwfl_report_offline_memory): Add libelf
	  as dependency.
	  * tests/dwfl-report-offline-memory.c: Add count_sections to
	  exercise Elf* from dwfl_report_offline_memory.
	  * tests/run-dwfl-report-offline-memory.sh: Add expected number
	  of sections to test invocations.

Signed-off-by: Aleksei Vetrov <vvvvvv@google.com>