Python: Use correct ptid in btrace recording

The user would always get the instruction_history and function_call_history
objects of the current thread, not the thread for which the gdb.Record object
was created.

The attached testcase fails without this patch and passes with the patch.

gdb/testsuite/gdb.python/py-record-btrace-threads.c

@@ -0,0 +1,58 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <pthread.h>
+
+static pthread_barrier_t barrier;
+static int dummy;
+
+static void *
+func1 (void *arg)
+{
+  pthread_barrier_wait (&barrier);
+  dummy = 1; /* bp1 */
+  pthread_barrier_wait (&barrier);
+  dummy = 1;
+  pthread_barrier_wait (&barrier);
+  return arg;
+}
+
+static void *
+func2 (void *arg)
+{
+  pthread_barrier_wait (&barrier);
+  dummy = 2;
+  pthread_barrier_wait (&barrier);
+  dummy = 2;
+  pthread_barrier_wait (&barrier); /* bp2 */
+  return arg;
+}
+
+int
+main (void)
+{
+  pthread_t thread;
+
+  pthread_barrier_init (&barrier, NULL, 2);
+
+  pthread_create (&thread, NULL, func2, NULL);
+  func1 (NULL);
+
+  pthread_join (thread, NULL);
+  pthread_barrier_destroy (&barrier);
+  return 0;
+}