[AArch64] Implement gdbarch_core_read_description

[AArch64] Implement gdbarch_core_read_description

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

This patch is an initial implementation of core_read_description method.  Future
features can use this to return selected description.

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