[aarch64] Add support for pointer authentication B key

[aarch64] Add support for pointer authentication B key

Armv8.3-A has another key used in pointer authentication called the
B-key (other than the A-key that is already supported). In order for
stack unwinders to work it is necessary to be able to identify frames
that have been signed with the B-key rather than the A-key and it was
felt that keeping this as an augmentation character in the CIE was the
best bet. The DWARF extensions for ARM therefore propose to add a new
augmentation character 'B' to the CIE augmentation string and the
corresponding cfi directive ".cfi_b_key_frame". I've made the relevant
changes to GAS and LD to add support for B-key unwinding, which required
modifying LD to check for 'B' in the augmentation string, adding the
".cfi_b_key_frame" directive to GAS and adding a "pauth_key" field to
GAS's fde_entry and cie_entry structs.

The pointer authentication instructions will behave as NOPs on
architectures that don't support them, and so a check for the
architecture being assembled for is not necessary since there will be no
behavioural difference between augmentation strings with and without the
'B' character on such architectures.

2018-12-05  Sam Tebbs  <sam.tebbs@arm.com>

bfd/
	* elf-eh-frame.c (_bfd_elf_parse_eh_frame): Add check for 'B'.

gas/
	* dw2gencfi.c (struct cie_entry): Add tc_cie_entry_extras invocation.
	(alloc_fde_entry): 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 and
	tc_cie_entry_init_extra invocation.
	(frch_cfi_data, cfa_save_data): Move to dwgencfi.h.
	* config/tc-aarch64.c (s_aarch64_cfi_b_key_frame): Declare.
	(md_pseudo_table): Add "cfi_b_key_frame".
	* config/tc-aarch64.h (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.h (struct fde_entry): Add tc_fde_entry_extras invocation.
	(pointer_auth_key): Define.
	(frch_cfi_data, cfa_save_data): Move from dwgencfi.c.
	* doc/c-aarch64.texi (.cfi_b_key_frame): Add documentation.
	* testsuite/gas/aarch64/(pac_ab_key.d, pac_ab_key.s): New file.