Add AVX(2) backend

Hi all,

This is a MR to add an AVX2 backend to Orc.

This work is a WIP derived from this branch: https://gitlab.freedesktop.org/amyspark/orc/-/commits/amyspark/sse-convert-load-to-temporary?ref_type=heads . I've refactored and dropped a lot of the WIP regarding codegen, in order to just target VEX-encoded SSE for review purposes. Once that's done, I'll begin/continue porting and adding where relevant the new AVX instructions from the other branch.

Basics

Orc internals documentation added
SSE codegen still works
AVX is detected
SSE instructions classified by presence in AVX(2)
VEX (orcc --assembly) codegen working
- I need to trace why the assembly comments look like coming from the SSE backend
VEX (binary) codegen working
Vector size enlargement
Addition of AVX(2) only instructions