Hide __LOAD_NEON_MODULE by using #[used] on a private static.

[eddyb]

See rust-lang/rust#39987. Still unstable, so you may want to wait for stabilization (if it's happening soon).

[RReverser]

@eddyb This is so great. Literally few minutes ago had to workaround this via std::sync::{Once, ONCE_INIT}.

[dherman]

Here's the tracking issue: rust-lang/rust#40289

When it lands in Rust stable we can use it ASAP.

[kjvalencik]

The Node-API backend uses a magic symbol name instead.