libdwfl, aarch64: Read PAC mask from core

libdwfl, aarch64: Read PAC mask from core

We need to read the PAC mask from a core file when debugging offline
as the information is still needed to demangle return addresses.

This commit pulls out the NT_ARM_PAC_MASK info from the core and feeds
it through to dwfl_thread_state_registers for each thread.

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