classes/cargo: fix up build CFLAGS being passed to Rust
classes/cargo: fix up build CFLAGS being passed to Rust

Firstly, BUILD_CC_ARCH is included in BUILD_CC (hence why we actually
need a BUILD_CC wrapper because it produces the space), so its
redundant (now with the wrapper using BUILD_CC). The RUST_BUILD_CFLAGS
variable doesn't need to be exported since its not actually used by
rustc/gcc-rs/cargo. Since its redundant and doesn't need to be
exported remove it and simplify.

meta-rust/meta-rust,starlab-io/meta-rust,cardoe/meta-rust,cardoe/meta-rust,cardoe/meta-rust,meta-rust/meta-rust,starlab-io/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,cardoe/meta-rust