use simd_saturating_{add, sub} on neon

[folkertdev]

The idea behind this is that e.g. miri and cranelift don't know all of the llvm intrinsics (and adding them is possible but a lot of extra work). Using the generic simd function means logic only needs to be implemented once for all targets.

The simd_saturating_* functions generate the same instructions for the saturating add/sub operations https://godbolt.org/z/zc35doTc6

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) some time within the next two weeks.

[folkertdev]

I don't think this idea works because of this LLVM issue I opened llvm/llvm-project#94463. So unless we can get that fixed (or somehow accept much worse codegen) this PR does not work

[Amanieu]

OK let's defer this for now until the LLVM issue is sorted.