Support AArch64 PAC elf-core register read

This adds support for reading AArch64 Pointer Authentication regset
from elf-core file. Also includes a test-case for the same. Furthermore
there is also a slight refactoring of RegisterContextPOSIXCore_arm64
members and constructor. linux-aarch64-pac.core file is generated using
lldb/test/API/functionalities/postmortem/elf-core/main.c with following
clang arguments:
-march=armv8.5-a -mbranch-protection=pac-ret+leaf -nostdlib -static -g

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D99941