aarch64: store signing key and signing method in DWARF _Unwind_FrameState

This patch is only a refactoring of the existing implementation
of PAuth and returned-address signing. The existing behavior is
preserved.

_Unwind_FrameState already contains several CIE and FDE information
(see the attributes below the comment "The information we care
about from the CIE/FDE" in libgcc/unwind-dw2.h).
The patch aims at moving the information from DWARF CIE (signing
key stored in the augmentation string) and FDE (the used signing
method) into _Unwind_FrameState along the already-stored CIE and
FDE information.
Note: those information have to be saved in frame_state_reg_info
instead of _Unwind_FrameState as they need to be savable by
DW_CFA_remember_state and restorable by DW_CFA_restore_state, that
both rely on the attribute "prev".

Those new information in _Unwind_FrameState simplifies the look-up
of the signing key when the return address is demangled. It also
allows future signing methods to be easily added.

_Unwind_FrameState is not a part of the public API of libunwind,
so the change is backward compatible.

A new architecture-specific handler MD_ARCH_EXTENSION_FRAME_INIT
allows to reset values (if needed) in the frame state and unwind
context before changing the frame state to the caller context.

A new architecture-specific handler MD_ARCH_EXTENSION_CIE_AUG_HANDLER
isolates the architecture-specific augmentation strings in AArch64
backend, and allows others architectures to reuse augmentation
strings that would have clashed with AArch64 DWARF extensions.

aarch64_demangle_return_addr, DW_CFA_AARCH64_negate_ra_state and
DW_CFA_val_expression cases in libgcc/unwind-dw2-execute_cfa.h
were documented to clarify where the value of the RA state register
is stored (FS and CONTEXT respectively).

libgcc/ChangeLog:

	* config/aarch64/aarch64-unwind.h
	(AARCH64_DWARF_RA_STATE_MASK): The mask for RA state register.
	(aarch64_ra_signing_method_t): The diversifiers used to sign a
	function's return address.
	(aarch64_pointer_auth_key): The key used to sign a function's
	return address.
	(aarch64_cie_signed_with_b_key): Deleted as the signing key is
	available now in _Unwind_FrameState.
	(MD_ARCH_EXTENSION_CIE_AUG_HANDLER): New CIE augmentation string
	handler for architecture extensions.
	(MD_ARCH_EXTENSION_FRAME_INIT): New architecture-extension
	initialization routine for DWARF frame state and context before
	execution of DWARF instructions.
	(aarch64_context_ra_state_get): Read RA state register from CONTEXT.
	(aarch64_ra_state_get): Read RA state register from FS.
	(aarch64_ra_state_set): Write RA state register into FS.
	(aarch64_ra_state_toggle): Toggle RA state register in FS.
	(aarch64_cie_aug_handler): Handler AArch64 augmentation strings.
	(aarch64_arch_extension_frame_init): Initialize defaults for the
	signing key (PAUTH_KEY_A), and RA state register (RA_no_signing).
	(aarch64_demangle_return_addr): Rely on the frame registers and
	the signing_key attribute in _Unwind_FrameState.
	* unwind-dw2-execute_cfa.h:
	Use the right alias DW_CFA_AARCH64_negate_ra_state for __aarch64__
	instead of DW_CFA_GNU_window_save.
	(DW_CFA_AARCH64_negate_ra_state): Save the signing method in RA
	state register. Toggle RA state register without resetting 'how'
	to REG_UNSAVED.
	* unwind-dw2.c:
	(extract_cie_info): Save the signing key in the current
	_Unwind_FrameState while parsing the augmentation data.
	(uw_frame_state_for): Reset some attributes related to architecture
	extensions in _Unwind_FrameState.
	(uw_update_context): Move authentication code to AArch64 unwinding.
	* unwind-dw2.h (enum register_rule): Give a name to the existing
	enum for the register rules, and replace 'unsigned char' by 'enum
	register_rule' to facilitate debugging in GDB.
	(_Unwind_FrameState): Add a new architecture-extension attribute
	to store the signing key.