use std::atomic to implement refcount class * oct-atomic.h, oct-atomic.c: New files. * liboctave/util/module.mk: Update. * dim-vector.h (dim_vector::increment_count, dim_vector::decrement_count): New functions. Use them instead of OCTAVE_ATOMIC_INCREMENT and OCTAVE_ATOMIC_DECREMENT macros. * Array.h (Array<T>::jit_ref_count): Return int, not pointer. Change all uses. * jit-typeinfo.h, jit-typeinfo.cc (jit_array::m_ref_count): Store integer, not pointer. Change all uses. * oct-refcount.h (OCTAVE_ATOMIC_INCREMENT, OCTAVE_ATOMIC_DECREMENT, OCTAVE_ATOMIC_POST_INCREMENT, OCTAVE_ATOMIC_POST_DECREMENT): Delete macros. (refcount::m_count): Now std::atomic<T> instead of T. (refcount::operator++, refcount::operator--): Define using std::atomic operators instead of increment macros. (refcount::get): Delete. (class refcount): Explicitly delete assignment operator and copy constructor. Explicitly use default destructor. Eliminate all uses of assignment as directly setting the reference count value apart from initialization should not be needed.