Use flexible target descriptors for aarch64

gdb/
	* aarch64-tdep.c (_initialize_aarch64_tdep): Remove init.
	* arch/aarch64.c (aarch64_create_target_description): Create
	new target description.
	* features/Makefile: Add new files.
	* features/aarch64.c: Remove file.
	* features/aarch64-core.c: New autogenerated file.
	* features/aarch64-fpu.c: New autogenerated file.
	* target-descriptions.c (maint_print_c_tdesc_cmd): Check for aarch64.

gdbserver/
	* linux-aarch64-ipa.c (initialize_low_tracepoint): Remove init.
	* linux-aarch64-low.c (initialize_low_arch): Remove init.
	* linux-aarch64-tdesc.c (aarch64_linux_read_description): Add init.