[PATCH 6/15] arm: Add pointer authentication for stack-unwinding runtime

This patch adds authentication for when the stack is unwound when an
exception is taken.  All the changes here are done to the runtime code
in libgcc's unwinder code for Arm target. All the changes are guarded
under defined (__ARM_FEATURE_PAC_DEFAULT) and activated only if the
+pacbti feature is switched on for the architecture. This means that
switching on the target feature via -march or -mcpu is sufficient and
-mbranch-protection need not be enabled. This ensures that the
unwinder is authenticated only if the PACBTI instructions are
available in the non-NOP space as it uses AUTG.  Just generating
PAC/AUT instructions using -mbranch-protection will not enable
authentication on the unwinder.

Pre-approved with the requested changes here
<https://gcc.gnu.org/pipermail/gcc-patches/2021-December/586555.html

>.

gcc/ChangeLog:

	* ginclude/unwind-arm-common.h (_Unwind_VRS_RegClass): Introduce
	new pseudo register class _UVRSC_PAC.

libgcc/ChangeLog:
	* config/arm/pr-support.c (__gnu_unwind_execute): Decode
	exception opcode (0xb4) for saving RA_AUTH_CODE and authenticate
	with AUTG if found.
	* config/arm/unwind-arm.c (struct pseudo_regs): New.
	(phase1_vrs): Introduce new field to store pseudo-reg state.
	(phase2_vrs): Likewise.
	(_Unwind_VRS_Get): Load pseudo register state from virtual reg set.
	(_Unwind_VRS_Set): Store pseudo register state to virtual reg set.
	(_Unwind_VRS_Pop): Load pseudo register value from stack into VRS.

Co-Authored-By: Tejas Belagod <tbelagod@arm.com>
Co-Authored-By: Srinath Parvathaneni <srinath.parvathaneni@arm.com>