Many build system improvements

[bjorn3]

• Share cross-compilation code between building and testing
• General cleanup of testing code
• Enable sparse cargo registry on CI
• Set core.autocrlf=false in the build system rather than on CI
• Fetch all cargo dependencies in ./y.rs prepare
• Improve readme and build system help message

Part of https://github.com/bjorn3/rustc_codegen_cranelift/issues/1290

[bjorn3]

No clue what causes the Cirrus CI FreeBSD failure. I mailed their support.

[bjorn3]

Cirrus CI's support pointed me to the fact that I was using an EOL version of FreeBSD. Updating it fixed the issue.