Auto merge of #4800 - msizanoen1:target-libs2, r=Manishearth

Only pass `--extern` flags for rlibs in target deps directory

It seems like there are proc macros/build scripts using serde: rust-lang/rust#66207 (comment)
This fix the build error by only passing `--extern` flags for rlibs in target deps directory (if available, otherwise host deps directory).

r? @Manishearth

changelog: none

tests/compile-test.rs

@@ -62,14 +62,8 @@ fn config(mode: &str, dir: PathBuf) -> compiletest::Config {
     // See https://github.com/rust-lang/rust-clippy/issues/4015.
     let needs_disambiguation = ["serde", "regex", "clippy_lints"];
     // This assumes that deps are compiled (they are for Cargo integration tests).
-    let deps = fs::read_dir(host_libs().join("deps")).unwrap();
-    let deps: Vec<_> = if let Some(target_libs) = target_libs() {
-        deps.chain(fs::read_dir(target_libs.join("deps")).unwrap()).collect()
-    } else {
-        deps.collect()
-    };
+    let deps = fs::read_dir(target_libs().unwrap_or_else(host_libs).join("deps")).unwrap();
     let disambiguated = deps
-        .into_iter()
         .filter_map(|dep| {
             let path = dep.ok()?.path();
             let name = path.file_name()?.to_string_lossy();