PAuthABI support for DWARF (Debugging and Unwinding)

[smithp35]

The PAuthABI work that we have done in the ABI has been restricted to the ELF specification. In the 31-07-2023 PAuthABI sync-up the topic of DWARF support came up as an area that will need expansion.

The current DWARF specification https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst#41dwarf-register-names has a pseudo register RA_SIGN_STATE that indicates that a register has been signed. This is sufficient for PACRET as the signing schema is implicit. It is also likely to be sufficient if the only operation is to strip signing. However for PAuthABI the unwinder and debugger may need to understand the signing schema at a more fine grained level, for example generating code for expression evaluation.

This issue is a placeholder for future improvements in the ABI. Most likely via a pauthabidwarf64 extension document like the ELF specification.