fix gdbstub support for multiple threads in usermode, v3

When debugging multi-threaded programs, QEMU's gdb stub would report the
correct number of threads (the qfThreadInfo and qsThreadInfo packets).
However, the stub was unable to actually switch between threads (the T
packet), since it would report every thread except the first as being
dead.  Furthermore, the stub relied upon cpu_index as a reliable means
of assigning IDs to the threads.  This was a bad idea; if you have this
sequence of events:

initial thread created
new thread #1
new thread #2
thread #1 exits
new thread #3

thread #3 will have the same cpu_index as thread #1, which would confuse
GDB.  (This problem is partly due to the remote protocol not having a
good way to send thread creation/destruction events.)

We fix this by using the host thread ID for the identifier passed to GDB
when debugging a multi-threaded userspace program.  The thread ID might
wrap, but the same sort of problems with wrapping thread IDs would come
up with debugging programs natively, so this doesn't represent a
problem.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>

exec.c

@@ -553,7 +553,7 @@ void cpu_exec_init(CPUState *env)
     penv = &first_cpu;
     cpu_index = 0;
     while (*penv != NULL) {
-        penv = (CPUState **)&(*penv)->next_cpu;
+        penv = &(*penv)->next_cpu;
         cpu_index++;
     }
     env->cpu_index = cpu_index;