Native LLVM atomics

In https://github.com/JuliaConcurrent/Atomix.jl/pull/39, where some IR with `atomicrmw` instructions is getting generated (as opposed to using AIR intrinsics), I noticed that the IR executes fine. Does that mean that Apple has been adding support for native LLVM atomics to Metal? If so, we should be able to simplify or even get rid of lots of the custom atomic stuff here.

```llvm
; │┌ @ /Users/tim/Julia/pkg/Atomix/src/core.jl:30 within `modify!`
; ││┌ @ /Users/tim/Julia/pkg/Atomix/src/references.jl:99 within `pointer` @ /Users/tim/Julia/pkg/Metal/src/device/array.jl:64
; │││┌ @ abstractarray.jl:1236 within `_memory_offset`
; ││││┌ @ int.jl:88 within `*`
%16 = shl nuw nsw i64 %14, 2
%17 = add nsw i64 %16, -4
; │││└└
; │││┌ @ /Users/tim/.julia/packages/LLVM/wMjUU/src/interop/pointer.jl:147 within `+`
; ││││┌ @ /Users/tim/.julia/packages/LLVM/wMjUU/src/interop/pointer.jl:114 within `add_ptr`
; │││││┌ @ /Users/tim/.julia/packages/LLVM/wMjUU/src/interop/pointer.jl:114 within `macro expansion` @ /Users/tim/.julia/packages/LLVM/wMjUU/src/interop/base.jl:39
%18 = getelementptr i8, i8 addrspace(1)* %.unpack, i64 %17
; ││└└└└
; ││ @ /Users/tim/Julia/pkg/Atomix/src/core.jl:33 within `modify!` @ /Users/tim/.julia/packages/UnsafeAtomicsLLVM/LPqS5/src/internal.jl:23 @ /Users/tim/.julia/packages/UnsafeAtomicsLLVM/LPqS5/src/internal.jl:23
; ││┌ @ /Users/tim/.julia/packages/UnsafeAtomicsLLVM/LPqS5/src/atomics.jl:399 within `atomic_pointermodify`
; │││┌ @ /Users/tim/.julia/packages/UnsafeAtomicsLLVM/LPqS5/src/atomics.jl:260 within `llvm_atomic_op`
; ││││┌ @ /Users/tim/.julia/packages/UnsafeAtomicsLLVM/LPqS5/src/atomics.jl:260 within `macro expansion` @ /Users/tim/.julia/packages/LLVM/wMjUU/src/interop/base.jl:39
%19 = bitcast i8 addrspace(1)* %18 to i32 addrspace(1)*
%20 = atomicrmw add i32 addrspace(1)* %19, i32 1 seq_cst, align 4
```