doc/book: fix tests for non-x86 architectures, such as aarch64
doc/book: fix tests for non-x86 architectures, such as aarch64

`rustdoc --test` gets confused when "main" exists for some architectures but not others.

graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust