MDEV-34297 get_rnd_value() of ib_counter_t is unnecessarily complex

The shared counter template ib_counter_t uses the function
my_timer_cycles() as a source of pseudo-random numbers to pick a shard.
On some platforms, my_timer_cycles() could return the constant value 0.

get_rnd_value(): Remove.

my_pseudo_random(): Implement as an alias of my_timer_cycles() or
a wrapper for pthread_self().

Reviewed by: Vladislav Vaintroub

include/my_rdtsc.h

@@ -177,10 +177,23 @@ static inline ulonglong my_timer_cycles(void)
   /* gethrtime may appear as either cycle or nanosecond counter */
   return (ulonglong) gethrtime();
 #else
+# define MY_TIMER_CYCLES_IS_ZERO
   return 0;
 #endif
 }
 
+#ifdef MY_TIMER_CYCLES_IS_ZERO
+static inline size_t my_pseudo_random(void)
+{
+  /* In some platforms, pthread_self() might return a structure
+  that cannot be converted to a number like this. Possible alternatives
+  could include gettid() or sched_getcpu(). */
+  return ((size_t) pthread_self()) / 16;
+}
+#else
+# define my_pseudo_random my_timer_cycles
+#endif
+
 /**
   A nanosecond timer.
   @return the current timer value, in nanoseconds.