rust: config: Fix missing CPU variant LD flags in Rust

This change fixes an issue where CPU variant LD flags from the cc toolchain were not applied to Rust libraries.
Rust linker flags are now in sync with cc.

Co-authored-by: Luca Stefani <luca.stefani.ge1@gmail.com>
Change-Id: Iaeecb363f98197f327b7bfccf03e5dc0f8252db8