libdwfl, aarch64: extend dwfl_thread_state_registers to handle PAC

libdwfl, aarch64: extend dwfl_thread_state_registers to handle PAC

On AArch64 systems with pointer authentication enabled, one needs to
know the PAC mask in order to unwind functions that employ PAC.

This patch extends dwfl_thread_state_registers to handle the PAC mask
information by introducing a special register -2. (-1 is used in a
similar manner already for handling the program counter).

The AArch64 linux process attach logic is also extended to query ptrace
for the PAC mask.

A subsequent patch will add support for retrieving the PAC mask from an
AArch64 linux core file.

Signed-off-by: Steve Capper <steve.capper@arm.com>