binutils: factorize handling of arch-specific DWARF augmentations

This patch factorizes the handling of architecture/machine specific
augmentation characters in CIEs.

Based on an idea proposed by Richard Earnshaw.

binutils/ChangeLog:

	* dwarf.c (is_mach_augmentation_ftype): New type.
	(is_mach_augmentation): New variable.
	(is_nomach_augmentation): New function.
	(is_aarch64_augmentation): Likewise.
	(init_dwarf_by_elf_machine_code): Set is_mach_augmentation as
	appropriate.
	(init_dwarf_by_bfd_arch_and_mach): Likewise.
	(read_cie): Handle architecture-specific augmentation characters
	in a generic way.