PR gdb/18717: internal error if non-leader thread exits process

PR gdb/18717: internal error if non-leader thread exits process

If a non-leader thread exits the process while all other threads are
ptrace-stopped, native gdb fails an assertion.  The test added by this
commit catches it:

 /home/pedro/gdb/mygit/build/../src/gdb/linux-nat.c:3198: internal-error: linux_nat_filter_event: Assertion `lp->resumed' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n)
 FAIL: gdb.threads/non-leader-exit-process.exp: program exits normally (GDB internal error)

The fix is just to remove the assertion.

With that out of the way, neither GDB not GDBserver handle this
perfectly though, so I'm adding a KFAIL:

 (gdb) continue
 Continuing.
 [Thread 0x7ffff7fc0700 (LWP 15350) exited]
 No unwaited-for children left.
 Couldn't get registers: No such process.
 (gdb) KFAIL: gdb.threads/non-ldr-exit.exp: program exits normally (PRMS: gdb/18717)

gdb/ChangeLog:
2015-07-24  Pedro Alves  <palves@redhat.com>

	PR gdb/18717
	* linux-nat.c (linux_nat_filter_event): Don't assert that the lwp
	is resumed, and extend the debug log.

gdb/testsuite/ChangeLog:
2015-07-24  Pedro Alves  <palves@redhat.com>

	PR gdb/18717
	* gdb.threads/non-ldr-exit.c: New file.
	* gdb.threads/non-ldr-exit.exp: New file.