Adjust gdb_continue_to_end for Windows

On Cygwin, supposely single-threaded programs are always
multi-threaded, due to the extra threads spawned by the Cygwin
runtime.  Because of that, any gdb_continue_to_end call that doesn't
specify "allow_extra" fails, like so:

 (gdb) PASS: gdb.base/langs.exp: show language at main
 continue
 Continuing.
 [Thread 16140.0x1fbc exited with code 0]
 [Thread 16140.0x2458 exited with code 0]
 [Thread 16140.0x3494 exited with code 0]
 [Inferior 1 (process 16140) exited normally]
 (gdb) FAIL: gdb.base/langs.exp: continue until exit at first session (the program exited)

Similarly, with this simple program compiled with MinGW:

 $ cat ~/sleeper.c
 #include <windows.h>

 int main ()
 {
   Sleep (2000);
   return 0;
 }

and with a MinGW GDB, I see:

 (gdb) start
 ...
 (gdb) info threads
   Id   Target Id           Frame
 * 1    Thread 15292.0x3850 main () at /home/alves/sleeper.c:5
   2    Thread 15292.0x3048 0x00007ff9630d2fb7 in ntdll!ZwWaitForWorkViaWorkerFactory () from C:\Windows\SYSTEM32\ntdll.dll
 (gdb) c
 Continuing.
 [Thread 15292.0x3850 exited with code 0]
 [Inferior 1 (process 15292) exited normally]
 (gdb)

This commit adjusts gdb_continue_to_end to expect the thread exited
messages, on Cygwin and MinGW.

Change-Id: I5e410a7252c11cd9ecea632f1e00c2a7fcd69098
Approved-By: Andrew Burgess <aburgess@redhat.com>