The fallback implementation was added for GCC 4.x as that didn't yet implement
std::is_trivially_copyable. However, by now we're using C++17 as our language
standard and that wasn't even implemented in GCC 4.x yet[^1]:
Some C++17 features are available since GCC 5, but support was experimental
and the ABI of C++17 features was not stable until GCC 9.
Hence, this became more or less dead code and can be removed.
[^1]: https://gcc.gnu.org/projects/cxx-status.html#cxx17
by not calling `std::atomic<T>::atomic(void)`.
After the latter the instance "does not contain a T object, and its only valid uses are destruction and initialization by std::atomic_init" which we don't call. So the only safe option is `std::atomic<T>::atomic(T)`.
https://en.cppreference.com/w/cpp/atomic/atomic/atomic
Apparently there was a reason for making the members of generated classes
atomic. However, this was only done for some types, others were still accessed
using non-atomic operations. For members of type T::Ptr (i.e. intrusive_ptr<T>),
this can result in a double free when multiple threads access the same variable
and at least one of them writes to the variable.
This commit makes use of std::atomic<T> for more T (it removes the additional
constraint sizeof(T) <= sizeof(void*)) and uses a type including a mutex for
load and store operations as a fallback.
