Currently, AArch64 only have one target description which is tdesc_aarch64.  So,
we haven't implemented any target description detetion mechanism for core file.

This patch is an initial implement of core_read_description method.  The "pauth"
feature or any future feature can use this to conditionally return selected
description.

gdb/
2017-08-09  Jiong Wang<jiong.wang@arm.com>

	* aarch64-linux-tdep.c (aarch64_linux_core_read_description): New
	function.
	(aarch64_linux_init_abi): Register gdbarch_core_read_description.