stdlib: Fix data race in __run_exit_handlers [BZ #27749]

stdlib: Fix data race in __run_exit_handlers [BZ #27749]

Keep __exit_funcs_lock almost all the time and unlock it only to execute
callbacks. This fixed two issues.

1. f->func.cxa was modified outside the lock with rare data race like:
	thread 0: __run_exit_handlers unlock __exit_funcs_lock
	thread 1: __internal_atexit locks __exit_funcs_lock
	thread 0: f->flavor = ef_free;
	thread 1: sees ef_free and use it as new
	thread 1: new->func.cxa.fn = (void (*) (void *, int)) func;
	thread 1: new->func.cxa.arg = arg;
	thread 1: new->flavor = ef_cxa;
	thread 0: cxafct = f->func.cxa.fn;  // it's wrong fn!
	thread 0: cxafct (f->func.cxa.arg, status);  // it's wrong arg!
	thread 0: goto restart;
	thread 0: call the same exit_function again as it's ef_cxa

2. Don't unlock in main while loop after *listp = cur->next. If *listp
   is NULL and __exit_funcs_done is false another thread may fail in
   __new_exitfn on assert (l != NULL):
	 thread 0: *listp = cur->next;  // It can be the last: *listp = NULL.
	 thread 0: __libc_lock_unlock
	 thread 1: __libc_lock_lock in __on_exit
	 thread 1: __new_exitfn
	 thread 1: if (__exit_funcs_done)  // false: thread 0 isn't there yet.
	 thread 1: l = *listp
	 thread 1: moves one and crashes on assert (l != NULL);

The test needs multiple iterations to consistently fail without the fix.

Fixes https://sourceware.org/bugzilla/show_bug.cgi?id=27749

Checked on x86_64-linux-gnu.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>