OpenCL overload of atomic_cmpxchg and atomic_add for float

atomic_cmpxchg_f32.cl

// See https://streamhpc.com/blog/2016-02-09/atomic-operations-for-floats-in-opencl-improved/

static float atomic_cmpxchg_f32(volatile __global float *p, float cmp, float val) {
	union {
		unsigned int u32;
		float        f32;
	} cmp_union, val_union, old_union;

	cmp_union.f32 = cmp;
	val_union.f32 = val;
	old_union.u32 = atomic_cmpxchg((volatile __global unsigned int *) p, cmp_union.u32, val_union.u32);
	return old_union.f32;
}

static float atomic_add_f32(volatile __global float *p, float val) {
	float found = *p;
	float expected;
	do {
		expected = found;
		found = atomic_cmpxchg_f32(p, expected, expected + val);
	} while (found != expected);
	return found;
}