This introduces DWARF support to enable unwinders to authenticate return
addresses signed using FEAT_PAuth_LR, where the value of PC is used as
an extra diversifier.

To achieve that, this proposes the following changes to the aadwarf64
document:
* Expanding usage of the `RA_SIGN_STATE` pseudo-register, allocating
  it's bit[1] to indicate wether the value of PC has been used for
  return address signing.
* Introducing a new vendor call frame instruction,
  `DW_CFA_AARCH64_negate_ra_state_with_pc`, which negates both bit[0]
  and bit[1] of `RA_SIGN_STATE` and instructs the unwinder to capture
  the current code location to be used when authenticating the return
  address.

Co-authored-by: Oliver Stannard <oliver.stannard@arm.com>

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