[dev.regabi] cmd/compile/internal: add internal ABI specification

This adds a document specifying the internal ABI (specifically the
calling convention). This document lives in the Go tree (rather than
the proposal repository) because the intent is for it to track the
reality in the compiler.

Updates #40724.

Change-Id: I583190080cd7d8cb1084f616fd1384d0f1f25725
Reviewed-on: https://go-review.googlesource.com/c/go/+/285292
Trust: Austin Clements <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>