Fix 'gcore' with exited threads

Program received signal SIGABRT, Aborted.
[...]
(gdb) gcore foobar
Couldn't get registers: No such process.
(gdb) info threads
[...]
(gdb) gcore foobar
Saved corefile foobar
(gdb)

gcore tries to access the exited thread:
[Thread 0x7ffff7fce700 (LWP 6895) exited]
ptrace(PTRACE_GETREGS, 6895, 0, 0x7fff18167dd0) = -1 ESRCH (No such process)

Without the TRY_CATCH protection testsuite FAILs for:
	gcore .../gdb/testsuite/gdb.threads/gcore-thread0.test
	Cannot find new threads: debugger service failed
	(gdb) FAIL: gdb.threads/gcore-thread.exp: save a zeroed-threads corefile
	+
	core .../gdb/testsuite/gdb.threads/gcore-thread0.test
	".../gdb/testsuite/gdb.threads/gcore-thread0.test" is not a core dump: File format not recognized
	(gdb) FAIL: gdb.threads/gcore-thread.exp: core0file: re-load generated corefile (bad file format)
Maybe the TRY_CATCH could be more inside update_thread_list().

Similar update_thread_list() call is IMO missing in procfs_make_note_section()
but I do not have where to verify that change.

gdb/ChangeLog
2014-08-21  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* linux-tdep.c (linux_corefile_thread_callback): Ignore THREAD_EXITED.
	(linux_make_corefile_notes): call update_thread_list, protected against
	exceptions.

gdb/testsuite/ChangeLog
2014-08-21  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.threads/gcore-stale-thread.c: New file.
	* gdb.threads/gcore-stale-thread.exp: New file.
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