Mach-O: add new defines and struct for darwin 14.5

Mach-O: add new defines and struct for darwin 14.5

include/mach-o/
	* arm64.h: New file.
	* unwind.h: Add arm64 encodings.
	* loader.h (bfd_mach_o_cpu_subtype): Add arm64 literals.
	(BFD_MACH_O_LC_ENCRYPTION_INFO_64, BFD_MACH_O_LC_LINKER_OPTIONS)
	(BFD_MACH_O_LC_LINKER_OPTIMIZATION_HINT)
	(BFD_MACH_O_LC_VERSION_MIN_WATCHOS): Define.
	* external.h (mach_o_encryption_info_64_command_external):
	(mach_o_linker_option_command_external)
	(mach_o_dyld_cache_header_external)
	(mach_o_dyld_cache_mapping_info_external)
	(mach_o_dyld_cache_image_info_external): New types.