mark internal functions linker private (#381)

Prevents unused functions in the emitted ELF object. Drive-by add a
missing test case (which misses a relocation under `-Oz` when all
internal functions are marked as private).

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/resolc/src/test_utils.rs

@@ -371,6 +371,7 @@ pub fn compile_blob_with_options(
         .object
         .as_str();
     let blob = hex::decode(bytecode).expect("hex encoding should always be valid");
+    assert_eq!(&blob[..3], b"PVM");
 
     PVM_BLOB_CACHE.lock().unwrap().insert(id, blob.clone());