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/llvm-context/src/polkavm/context/function/runtime/deploy_code.rs

@@ -36,7 +36,7 @@ where
             runtime::FUNCTION_DEPLOY_CODE,
             function_type,
             0,
-            Some(inkwell::module::Linkage::External),
+            Some(inkwell::module::Linkage::Private),
             None,
         )?;