Preserve all mapping symbols in ARM and AArch64 object files.

bfd	* elfnn-aarch64.c (is_aarch64_mapping_symbol): New function.
	Returns TRUE for AArch64 mapping symbols.
	(elfNN_aarch64_backend_symbol_processing): New function.  Marks
	mapping symbols as precious in object files so that they will not
	be stripped.
	(elf_backend_symbol_processing): Define.

	* elf32-arm.c (is_arm_mapping_symbol): New function.  Returns TRUE
	for ARM mapping symbols.
	(elf32_arm_backend_symbol_processing): Make use of the new function.