Enable SVE for GDB

This patch enables SVE support for GDB by reading the VQ when
creating a target description.

It also ensures that SVE is taken into account when creating
the tdep structure, and stores the current VQ value directly
in tdep.

With this patch, gdb on an aarch64 system with SVE will now detect
SVE. The SVE registers will be displayed (but the contents will be
invalid). The following patches fill out the contents.

2018-05-101 Alan Hayward  <alan.hayward@arm.com>

	* 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.