MDEV-31871: maria-install-db fails on MacOS

mariadb-install crashes on start when the static variable debug_sync_global
of the class st_debug_sync_globals is initialized by constructor.

Definition of the class st_debug_sync_globals has a data member of the type
Hash_set whose implementation depends on thread-specific data associated
with the key THD_KEY_mysys. This dependency results from constructor of
the class Hash_set that runs my_hash_init2() which in turn invokes my_malloc.
The thread-specific data value associated with the key THD_KEY_mysys is used
by the function sf_malloc to get id of the current thread. The key
THD_KEY_mysys is defined as static variable at my_thr_init.c initialized
with the value -1. Proper initialization of the key THD_KEY_mysys is done
with the library call pthread_key_create but it happens at the  my_init()
that called much later after the first time the THD_KEY_mysys() has been
invoked. In according with Single Unix Specification, the effect of calling
pthread_setspecific() or pthread_getspecific() with a key value not obtained
from pthread_key_create() is undefined. That is the reason why mariadb-install
crashes on MacOS.

To fix the issue, the static variable debug_sync_global is converted to
a pointer to the class st_debug_sync_globals and its instantiation is done
explicitly at the function debug_sync_init() that is called at right time.

This is the follow-up patch to the commits
  8885225d
  f6ecadfe
where was introduced a statically instantiated object debug_sync_global of
the structure st_debug_sync_globals and the key THR_KEY_mysys for this
thread-specific data was initialized with the value -1.