Instead of using atomic<> to access global function pointers, use raw
pointers and atomic_ref to access them safely in multi-threaded builds.
This allows to ensure constant initialization of the function pointers,
even in C++03. This also solves the problem of undefined dynamic
initialization order that we previously tried to solve with the
init_priority attribute. The attribute turns out to not work if the
pointers were raw pointers (in single-threaded builds). It is also
not supported by Intel Compiler and possibly other, which required
us to avoid using the function pointer for fill_random.
The resulting code should be simpler and more portable. In order to
check for C++20 std::atomic_ref availability, the older check for <atomic>
header was replaced with a check for std::atomic_ref. If not available,
we're using Boost.Atomic, as before.
