gdbserver: When attaching, add process before lwps

The recent BP/WP changes for AArch64 swapping the order in add_lwp()
so that the process was added before the lwp. This was due to the lwp
creation requiring the process data.

This also needs changing in linux_attach().

Fixes gdb.server/ext-attach.exp on Aarch64.

(This regression was hidden due to the racy nature of the gdb.server
tests - now they are no longer racy it'll be easier to spot. Also
checked X86).

gdb/gdbserver/ChangeLog:

2019-01-25  Alan Hayward  <alan.hayward@arm.com>

	* linux-low.c (linux_attach): Add process before lwp.

gdb/gdbserver/linux-low.c

@@ -1188,6 +1188,8 @@ linux_attach (unsigned long pid)
   ptid_t ptid = ptid_t (pid, pid, 0);
   int err;
 
+  proc = linux_add_process (pid, 1);
+
   /* Attach to PID.  We will check for other threads
      soon.  */
   err = linux_attach_lwp (ptid);
@@ -1198,8 +1200,6 @@ linux_attach (unsigned long pid)
       error ("Cannot attach to process %ld: %s", pid, reason.c_str ());
     }
 
-  proc = linux_add_process (pid, 1);
-
   /* Don't ignore the initial SIGSTOP if we just attached to this
      process.  It will be collected by wait shortly.  */
   initial_thread = find_thread_ptid (ptid_t (pid, pid, 0));