Move aarch64 CIE code to aarch64 backend

Move aarch64 CIE code to aarch64 backend

This commit moves all aarch64-specific code to deal with CIE structure
introduced in 3a67e1a6b4430374f3073e51bb19347d4c421cfe from
target-independent files to the aarch64 backend.

2018-12-13  Sam Tebbs  <sam.tebbs@arm.com>

binutils/
	* dwarf.c (read_cie):  Add check for 'B'.

gas/
	* config/tc-aarch64.h (enum pointer_auth_key,
	tc_fde_entry_extras, tc_cie_entry_extras, tc_fde_entry_init_extra,
	tc_output_cie_extra, tc_cie_fde_equivalent_extra,
	tc_cie_entry_init_extra): Define.
	* dw2gencfi.c (struct cie_entry): Add tc_cie_entry_extras invocation.
	(alloc_fde_entry, select_cie_for_fde): Add tc_fde_entry_init_extra
	invocation.
	(output_cie): Add tc_output_cie_extra invocation.
	(select_cie_for_fde): Add tc_cie_fde_equivalent_extra invocation.
	* dw2gencfi.h (enum pointer_auth_key): Move to config/tc-aarch64.h.
	(struct fde_entry): Add tc_fde_entry_extras invocation