KernelAbstractions: add Atomix back-end

[maleadt]

See https://github.com/JuliaConcurrent/Atomix.jl/tree/main/lib/AtomixCUDA and JuliaGPU/CUDA.jl#1790

[maleadt]

The alternative is that we rely on LLVM atomics, and lower them to something SPIR-V/OpenCL compatible in GPUCompiler.

[michel2323]

@maleadt This is resolved by SPIRVIntrinsics.jl right?