gdb/testsuite: fix race condition in gdb.replay/missing-thread.c

I get frequent failures in gdb.replay/missing-thread.exp, starting with:

    FAIL: gdb.replay/missing-thread.exp: non_stop=on: record_initial_logfile: continue (timeout)

I tracked this down to a race condition in the test program, causing it
not to generate the expected SIGTRAP.  This can be reproduced by adding
a sleep(1) just after the pthread_create.

The expected behavior is:

 - The main thread starts a second thread
 - The main thread blocks on pthread_cond_wait, waiting to be notified
   by the second thread (to make sure the second thread had time to
   start)
 - The second thread notifies the main thread using pthread_cond_signal
 - The main thread sends a SIGTRAP to the second thread

However, this can happen:

 - The main thread starts a second thread
 - The second thread calls pthread_cond_signal, while no one is waiting
   to the condvar
 - The main thread blocks on pthread_cond_wait forever

Fix it by introducing a separate boolean predicate protected by the
shared mutex, and looping until it is true.  This is the way
pthread_cond_wait is meant to be used. From pthread_cond_wait(3):

    When using condition variables there is always a boolean predicate
    involving shared variables associated with each condition wait that
    is true if the thread should proceed. Spurious wakeups from the
    pthread_cond_wait() or pthread_cond_timedwait() functions may occur.
    Since the return from pthread_cond_wait() or
    pthread_cond_timedwait() does not imply anything about the value of
    this predicate, the predicate should be re-evaluated upon such
    return.

Finally, make things static, just out of principle, and fix minor
formatting issues.

Change-Id: I62ba2085a2d506dc3d91e32cd5de48c43a3ff55e
Approved-By: Tom Tromey <tom@tromey.com>