Aarch64 SVE: Support changing vector lengths in GDB

Aarch64 SVE: Support changing vector lengths in GDB

Override the thread_architecture method to place the vector length in the
tdep_info and then find using info. Do not set this as a pointer as
this will cause issues in later patches.

2018-11-07  Alan Hayward  <alan.hayward@arm.com>

gdb/
	* aarch64-linux-nat.c
	(aarch64_linux_nat_target::thread_architecture): Add override.
	* aarch64-tdep.c (aarch64_get_tdesc_vq): Check for nullptr.
	(aarch64_gdbarch_init): Ensure differemt tdesc for each VQ.