Fix threadapply test

Fix threadapply test

The current test case leaves detached processes running at the end of
the test.  This patch changes the test to use a barrier wait to ensure all
processes exit cleanly at the end of the tests.

gdb/testsuite/ChangeLog:

2021-06-02  Carl Love  <cel@us.ibm.com>

	* gdb.threads/threadapply.c: Add global mybarrier.
	(main): Add pthread_barrier_init.
	(thread_function): Replace while loop with myp increment and
	pthread_barrier_wait.