Replace calls to abort (in readelf) with informative error messages.

Replace calls to abort (in readelf) with informative error messages.

	PR binutils/17531
	* readelf.c (get_machine_flags): Replace call to abort with a
	warning message and a return value.
	(get_elf_section_flags): Likewise.
	(get_symbol_visibility): Likewise.
	(get_ia64_symbol_other): Likewise.
	(get_ia64_symbol_other): Likewise.
	(is_32bit_abs_reloc): Likewise.
	(apply_relocations): Likewise.
	(display_arm_attribute): Likewise.