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).

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

crates/integration/src/tests.rs

@@ -61,6 +61,7 @@ test_spec!(delegate_no_contract, "DelegateCaller", "DelegateCaller.sol");
 test_spec!(function_type, "FunctionType", "FunctionType.sol");
 test_spec!(layout_at, "LayoutAt", "LayoutAt.sol");
 test_spec!(shift_arithmetic_right, "SAR", "SAR.sol");
+test_spec!(add_mod_mul_mod, "AddModMulModTester", "AddModMulMod.sol");
 
 fn instantiate(path: &str, contract: &str) -> Vec<SpecsAction> {
     vec![Instantiate {