Update or remove the list of architectures in the reference

[amartini51]

Location

https://docs.swift.org/swift-book/documentation/the-swift-programming-language/statements#Conditional-Compilation-Block

Description

The list of architectures next to the arch() platform condition might be out of date.

Correction

Is arm64_32 officially supported? If so, it should be added.

Alternately, it's been suggested that the Swift project itself, and not TSPL, should own this list. so maybe we should just remove it and handle architectures like we do version numbers. If possible, we should link to a page on Swift.org that has a list of official (and unofficial?) support.