@atomic is fragile due to bypassing getindex machinery

Compare
```
julia> @cuda ((a)->(a[2] = 42; nothing))(CuArray{Int}(undef, 1))

julia> using CUDAdrv

julia> CUDAdrv.synchronize()
ERROR: a exception was thrown during kernel execution.
Run Julia on debug level 2 for device stack traces.
ERROR: CUDA error: an illegal instruction was encountered (code #715, ERROR_ILLEGAL_INSTRUCTION)
Stacktrace:
[1] macro expansion at /home/tim/Julia/pkg/CUDAdrv/src/base.jl:145 [inlined]
[2] synchronize(::CuContext) at /home/tim/Julia/pkg/CUDAdrv/src/context.jl:196 (repeats 2 times)
[3] top-level scope at REPL[8]:1
```

vs

```
julia> @cuda ((a)->(CUDAnative.@atomic a[2] += 42; nothing))(CuArray{Int}(undef, 1))

julia> CUDAdrv.synchronize()

julia>
```