Enable SVE support for GDB by reading the VQ when creating a
target description.

Also ensurse that SVE is taken into account when creating
the tdep structure, and store the current VQ value directly in tdep.

gdb/
	* aarch64-linux-nat.c (aarch64_linux_read_description): Support SVE.
	* aarch64-tdep.c (aarch64_get_tdesc_vq): New function.
	(aarch64_gdbarch_init): Check for SVE.
	* aarch64-tdep.h (gdbarch_tdep::has_sve): New function.