AArch64 SVE: Check for vector length change when getting gdbarch

AArch64 SVE: Check for vector length change when getting gdbarch

Override the thread_architecture method, similar to SPU.  If the vector
length has changed, then find the arch using info, making sure the vector
length is passed down to the init routine.

In the init routine, ensure the arch has the correct vector length.

Example output. Program is stopped in thread 2, just before it calls prctl
to change the vector length

(gdb) info threads
  Id   Target Id                                     Frame
  1    Thread 0xffffbf6f4000 (LWP 3188) "sve_change" 0x0000ffffbf6ae130 in pthread_join ()
* 2    Thread 0xffffbf55e200 (LWP 3189) "sve_change" thread1 (arg=0xfeedface) at sve_change_size.c:28
(gdb) print $vg
$1 = 8
(gdb) print $z0.s.u
$2 = {623191333, 623191333, 623191333, 623191333, 0 <repeats 12 times>}
(gdb) n
29	  int ret = prctl(PR_SVE_SET_VL, vl/2);
(gdb) n
30	  printf ("Changed: ret\n", ret);
(gdb) print $vg
$4 = 4
(gdb) print $z0.s.u
$5 = {623191333, 623191333, 623191333, 623191333, 0, 0, 0, 0}
(gdb) thr 1
[Switching to thread 1 (Thread 0xffffbf6f4000 (LWP 3181))]
(gdb) print $vg
$6 = 8
(gdb) print $z0.s.u
$7 = {623191333, 623191333, 623191333, 623191333, 0 <repeats 12 times>}

gdb/ChangeLog:

	* aarch64-linux-nat.c
	(aarch64_linux_nat_target::thread_architecture): Add override.
	* aarch64-tdep.c (aarch64_gdbarch_init): Ensure different tdesc for
	each VQ.