Fix thread's gdbarch when SVE vector length changes

When the inferior program changes the SVE length, GDB can stop tracking
some registers as it obtains the new gdbarch that corresponds to the
updated length:

  Breakpoint 1, do_sve_ioctl_test () at sve-ioctls.c:44
  44              res = prctl(PR_SVE_SET_VL, i, 0, 0, 0, 0);
  (gdb) print i
  $2 = 32
  (gdb) info registers
          ⋮
  [ snip registers x0 to x30 ]
          ⋮
  sp             0xffffffffeff0      0xffffffffeff0
  pc             0xaaaaaaaaa8ac      0xaaaaaaaaa8ac <do_sve_ioctl_test+112>
  cpsr           0x60000000          [ EL=0 BTYPE=0 C Z ]
  fpsr           0x0                 0
  fpcr           0x0                 0
  vg             0x8                 8
  tpidr          0xfffff7fcb320      0xfffff7fcb320
  (gdb) next
  45              if (res < 0) {
  (gdb) info registers
          ⋮
  [ snip registers x0 to x30 ]
          ⋮
  sp             0xffffffffeff0      0xffffffffeff0
  pc             0xaaaaaaaaa8cc      0xaaaaaaaaa8cc <do_sve_ioctl_test+144>
  cpsr           0x200000            [ EL=0 BTYPE=0 SS ]
  fpsr           0x0                 0
  fpcr           0x0                 0
  vg             0x4                 4
  (gdb)

Notice that register tpidr disappeared when vg (which holds the vector
length) changed from 8 to 4.  The tpidr register is provided by the
org.gnu.gdb.aarch64.tls feature.

This happens because the code that searches for a new gdbarch to match the
new vector length in aarch64_linux_nat_target::thread_architecture doesn't
take into account the features present in the target description associated
with the previous gdbarch.  This patch makes it do that.

Since the id member of struct gdbarch_info is now unused, it's removed.

gdb/gdbarch.h

@@ -251,17 +251,8 @@ struct gdbarch_info
 
   bfd *abfd = nullptr;
 
-  union
-    {
-      /* Architecture-specific target description data.  Numerous targets
-	 need only this, so give them an easy way to hold it.  */
-      struct tdesc_arch_data *tdesc_data;
-
-      /* SPU file system ID.  This is a single integer, so using the
-	 generic form would only complicate code.  Other targets may
-	 reuse this member if suitable.  */
-      int *id;
-    };
+  /* Architecture-specific target description data.  */
+  struct tdesc_arch_data *tdesc_data;
 
   enum gdb_osabi osabi = GDB_OSABI_UNKNOWN;