Support for passing synchronization scopes

[maleadt]

Currently, only LLVM.Internal.atomic_pointermodify takes a syncscope argument; probably all UnsafeAtomics APIs do.

@pxl-th How are you setting custom syncscopes?

[pxl-th]

At the moment I'm just setting syncscope to agent for all FP +- atomic ops:
https://github.com/JuliaGPU/AMDGPU.jl/blob/57d8324533209b456e789897c18a10b021afc090/src/AMDGPU.jl#L149

[vchuravy]

After #20 we can now do UnsafeAtomics.modify!(ptr, op, x, ord, UnsafeAtomics.Internal.LLVMSyncScope{:agent}())

[vchuravy]

JuliaGPU/AMDGPU.jl#709