Add announcement for `rust-lld` on linux nightlies

[lqd]

This PR is the companion announcement post needed for https://github.com/rust-lang/rust/pull/124129, about switching the default linker on linux nightlies to rust-lld, our "self-contained lld".

The publication date is just a random day next week, just to leave time for comments here and on the rustc PR. I'm also not sure if this should be published on Inside Rust (like I've done here), or the main blog.

cc @Mark-Simulacrum and @rust-lang/wg-compiler-performance in particular