gdb: use all_threads() in for_each_thread()

In for_each_thread(), we use all_threads_safe(), which allows deleting the
thread.  This it not needed in the cases where it is currently used and it
causes the iterator to get corrupted when threads are deleted in a nested
all_threads_safe() iteration.

One such scenario is as follows:
- remote debugging in non-stop mode
- one thread has hit a breakpoint
- one thread exited but GDB does not know about it, yet
- resume with 'continue -a'
- the continue command uses for_each_thread() to proceed threads individually
- the stopped thread starts an inline step-over (displaced stepping disabled)
- update_thread_list() learns about the exit and deletes the thread
- the safe iterator's m_next still points to the deleted thread

Approved-By: Tom Tromey <tom@tromey.com>