Add note about cross compiling via nightly

[Noxsios]

It is possible to cross compile w/ the +nightly targets.

ex.

rustup +nightly target add x86_64-unknown-linux-musl
cargo +nightly build -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort \
    --target x86_64-unknown-linux-musl --release

Will work, this can be added as an addendum to the Optimize libstd and Remove Panic String sections.

Discovered from: rust-lang/wg-cargo-std-aware#76

[johnthagen]

Thanks for the post.

Does cross compiling in some way reduce the final binary size? While a great feature, my goal is to keep min-sized-rust focused exclusively on binary size reduction techniques rather than trying to mention all possible Rust features.

[Noxsios]

AFAIK, compiling for aarch64-apple-darwin (M1 Mac) binaries are slightly smaller, but not by a substantial amount. Just wanted to add this addendum for people looking to cross compile while also running your provided flags.