clang/AMDGPU: Emit atomicrmw from ds_fadd builtins

We should have done this for the f32/f64 case a long time ago. Now that
codegen handles atomicrmw selection for the v2f16/v2bf16 case, start emitting
it instead.

This also does upgrade the behavior to respect a volatile qualified pointer,
which was previously ignored (for the cases that don't have an explicit
volatile argument).