Add test for fetching TLS from core file

A correct PID is needed by `libthread_db' library supplied with
glibc repository revisions before commit c579f48edba8 ("Remove
cached PID/TID in clone") or versions before 2.25 release for
GDB to fetch value of TLS variable from core file.  On MIPS
platforms it was omitted and fetching value of TLS variable was not
available.

This adds a new test in order to be sure if GDB on native platforms
can successfully fetch value of TLS variable.

gdb/testsuite:

	* gdb.threads/tls-core.c: New file.
	* gdb.threads/tls-core.exp: Likewise.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-08  Djordje Todorovic  <djordje.todorovic@rt-rk.com>
+
+	* gdb.threads/tls-core.c: New file.
+	* gdb.threads/tls-core.exp: Likewise.
+
 2017-11-08   Pedro Alves  <palves@redhat.com>
 
 	* gdb.ada/complete.exp (p <Exported_Capitalized>): New test.

gdb/testsuite/gdb.threads/tls-core.c

@@ -0,0 +1,37 @@
+/* This test 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>
+
+int __thread foo = 0xdeadbeef;
+
+static void *
+thread_proc (void *arg)
+{
+  return arg;
+}
+
+int
+main (void)
+{
+  pthread_t thread;
+
+  pthread_create (&thread, NULL, thread_proc, NULL);
+  pthread_join (thread, NULL);
+
+  return 0;
+}

gdb/testsuite/gdb.threads/tls-core.exp

@@ -0,0 +1,56 @@
+# 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/>.
+
+standard_testfile
+
+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" \
+	executable { debug }] != "" } {
+    return -1
+}
+
+
+clean_restart ${binfile}
+
+runto thread_proc
+
+#
+# Generate corefile.
+#
+set corefile [standard_output_file gcore.test]
+set core_supported [gdb_gcore_cmd "$corefile" "save a corefile"]
+if {!$core_supported} {
+    return 0
+}
+
+#
+# Restart gdb and load generated corefile.
+#
+clean_restart ${binfile}
+
+set core_loaded [gdb_core_cmd "$corefile" "load generated corefile"]
+if { $core_loaded != 1 } {
+    # No use proceeding from here.
+    return 0
+}
+
+# This fails in cross-debugging due to the use of native `libthread_db'.
+if {![string match $host_triplet $target_triplet]} {
+    setup_kfail "threads/22381" "*-*-*"
+}
+gdb_test "p/x foo" \
+	"\\$\[0-9]+ = 0xdeadbeef" \
+	"print thread-local storage variable"
+
+gdb_exit