In commit 99bd2260 (MDEV-31558)
we wrongly thought that there would be minimal overhead for accessing
a thread-local variable mariadb_stats.

It turns out that in C++11, each access to an extern thread_local
variable requires conditionally invoking an initialization function.
In fact, the initializer expression of mariadb_stats is dynamic, and
those calls were actually unavoidable.

In C++20, one could declare constinit thread_local variables, but
the address of a thread_local variable (&mariadb_dummy_stats) is not
a compile-time constant. We did not want to declare mariadb_dummy_stats
without thread_local, because then the dummy accesses could lead to
cache line contention between threads.

mariadb_stats: Declare as __thread or __declspec(thread) so that
there will be no dynamic initialization, but zero-initialization.

mariadb_dummy_stats: Remove. It is a lesser evil to let
the environment perform zero-initialization and check if
!mariadb_stats.

Reviewed by: Sergei Petrunia