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/yul/src/parser/statement/function_definition.rs

@@ -215,7 +215,7 @@ impl PolkaVMWriteLLVM for FunctionDefinition {
             self.identifier.as_str(),
             function_type,
             self.result.len(),
-            Some(inkwell::module::Linkage::External),
+            Some(inkwell::module::Linkage::Private),
             Some((self.location.line, self.location.column)),
         )?;
         PolkaVMFunction::set_attributes(