Bug #28284: Test "mysqlslap" reports "out of memory"

When locking a "fast" mutex a static variable cpu_count 
was used as a flag to initialize itself on the first usage 
by calling sysconf() and setting non-zero value.
This is not thread and optimization safe on some 
platforms. That's why the global initialization needs 
to be done once in a designated function.
This will also speed up the usage (by a small bit) 
because it won't have to check if it's initialized on
every call.

Fixed by moving the fast mutexes initialization out of 
my_pthread_fastmutex_lock() to fastmutex_global_init()
and call it from my_init()


include/my_pthread.h:
  Bug #28284: move the fast mutexes initialization out of 
  my_pthread_fastmutex_lock() to fastmutex_global_init()
  and call it from my_init()
mysys/my_init.c:
  Bug #28284: move the fast mutexes initialization out of 
  my_pthread_fastmutex_lock() to fastmutex_global_init()
  and call it from my_init()
mysys/thr_mutex.c:
  Bug #28284: move the fast mutexes initialization out of 
  my_pthread_fastmutex_lock() to fastmutex_global_init()
  and call it from my_init()

include/my_pthread.h

@@ -538,6 +538,7 @@ typedef struct st_my_pthread_fastmutex_t
   pthread_mutex_t mutex;
   uint spins;
 } my_pthread_fastmutex_t;
+void fastmutex_global_init(void);
 
 int my_pthread_fastmutex_init(my_pthread_fastmutex_t *mp, 
                               const pthread_mutexattr_t *attr);