Improve performance of `global.{set,get}` with externref

First implemented in https://github.com/bytecodealliance/wasmtime/pull/1969 the easiest thing at the time was calling out to intrinsic functions to implement `global.{get,set}`, which means that there are no JIT fast paths for these operations. At the time of this writing there's no toolchain that uses `global.{get,set}` so we don't have a ton of motivation to make this fast, but I wanted to file this issue to make sure we don't forget about it and in case it comes up in the future!

For now it's likely safe to not worry about the performance here, but if this becomes a hot spot in the future this can be a point of coordination!

Inizia esaminando PR 1969 e l’implementazione di global.{get,set} per externref. Individua dove le chiamate intrinseche entrano nel JIT e determina quale lavoro sul fast-path e quali misurazioni delle prestazioni sarebbero necessari; il lavoro sarebbe considerato completato quando queste operazioni non dipenderanno più dallo slow path una volta diventate un hotspot rilevante.

Scritto dal modello di indicizzazione a partire dal testo della issue.