How should we handle dynamic vector ABIs?

[workingjubilee]

The primary unresolved question of #131800 I would say is this edge-case. Opening this marker issue for now, I will explain a bit later but basically: we understand most of the factors here but we need to account for what we actually want to do.

AArch64

• Scalable Vector Extension
• Scalable Matrix Extension: also see How should we handle matrix ABIs? #133144

RISCV

• "V" extension: https://github.com/riscvarchive/riscv-v-spec/blob/master/v-spec.adoc