Mention the u128/i128 ABI change

Author: cuviper

posts/2024-05-02-Rust-1.78.0.md

@@ -137,6 +137,7 @@ These APIs are now stable in const contexts:
   - `i686-pc-windows-gnu`
   - `x86_64-pc-windows-gnullvm`
   - `i686-pc-windows-gnullvm`
+* Rust 1.78 has upgraded its bundled LLVM to version 18, completing the announced [`u128`/`i128` ABI change](https://blog.rust-lang.org/2024/03/30/i128-layout-update.html) for x86-32 and x86-64 targets. Distributors that use their own LLVM older than 18 may still face the calling convention bugs mentioned in that post.
 
 ### Other changes