aarch64: use portable SIMD intrinsics where possible

[RalfJung]

We have a number of portable SIMD intrinsics in core::intrinsics::simd. The advantage of using those is that they have to only be implemented once by non-LLVM backends (Miri, cranelift, gcc) and then they work consistently across all architectures. The x86 parts of stdarch thankfully make good use of them, which heavily reduces the number of intrinsics Miri and the other backends have to special-case by hand.

It would be good to do the same for aarch64. AFAIK aarch64 has completely standard float semantics including the behavior on subnormals, so there shouldn't be a reason to use architecture-specific intrinsics. (arm-32 is a different case since the neon intrinsics there flush subnormals to 0, which makes it technically not correct to use the portable intrinsics. But also at least LLVM doesn't implement the portable intrinsics properly on arm-32 so they actually do flush subnormals, except if optimizations kick in in which case we can get unsoundness.)