Preserve machine dependent section header flags when reading ELF object file.

Preserve machine dependent section header flags when reading ELF object file.
	* elf-bfd.h (struct elf_backend_data): Add elf_backend_section_flags
	field.
	* elf.c (_bfd_elf_make_section_from_shdr): Call the
	elf_backend_section_flags function.
	* elf64-ia64.c (elf64_ia64_section_from_shdr): Delete flag conversion
	code.
	(elf64_ia64_section_flags): New function containing flag conversion
	code.
	(elf_backend_section_flags): Define to elf64_ia64_section_flags.
	* elfxx-target.h (elf_backend_section_flags): Define.
	(elfNN_bed): Initialize elf_backend_section_flags field.