Fix the race in gdb.threads/staticthreads.exp

The code in gdb.threads/staticthreads.exp about checking the value of
tlsvar in main thread is racy, because when child thread hits
breakpoint, the main thread may not go into pthread_join yet, and
may not be unwind to main.

This patch is to move the line setting breakpoint on after sem_wait,
so that the child thread will hit breakpoint after main thread calls
sem_post.  IOW, when child thread hits breakpoint, the main thread is
in either sem_post or pthread_join.  "up 10" can unwind main thread to
main.

gdb/testsuite:

2014-06-06  Yao Qi  <yao@codesourcery.com>

	* gdb.threads/staticthreads.c (thread_function): Move the line
	setting breakpoint on forward.
	* gdb.threads/staticthreads.exp: Update comments.