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>

Makefile

@@ -69,7 +69,7 @@ test-resolc: install
 	cargo test --package resolc
 
 test-workspace: install
-	cargo test --workspace --exclude revive-llvm-builder --doc
+	cargo test --workspace --exclude revive-llvm-builder
 
 test-wasm: install-wasm
 	npm run test:wasm