[AVR][No Upstream] Cherry-pick workaround for rust-lang#39880

This shouldn't need to be in tree, but I'm committing it anyway because
it's a headache when compiling with a separate LLVM root.

Author: dylanmckay

src/librustc_llvm/lib.rs

@@ -24,6 +24,9 @@
 #![feature(link_args)]
 #![feature(static_nobundle)]
 
+// Workaround for rust-lang/rust#39880
+#[link(name = "ffi")] extern {}
+
 // See librustc_cratesio_shim/Cargo.toml for a comment explaining this.
 #[allow(unused_extern_crates)]
 extern crate rustc_cratesio_shim;