Arm SVE intrinsics

[jedbrown]

Arm SVE intrinsics are vector-length agnostic, so represent a nontrivial difference in strategy from Intel intrinsics. An example shows how one might program with these intrinsics:

https://developer.arm.com/documentation/100891/0612/coding-considerations/using-sve-intrinsics-directly-in-your-c-code

Scatters and gathers are supported.

The new Neoverse V1 and N2 will support SVE, so we can expect to see it appearing in more server environments (i.e., not just A64FX). https://fuse.wikichip.org/news/4564/arm-updates-its-neoverse-roadmap-new-bfloat16-sve-support/