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/runtime.rs

@@ -30,7 +30,7 @@ pub trait RuntimeFunction {
             Self::NAME,
             Self::r#type(context),
             0,
-            Some(inkwell::module::Linkage::External),
+            Some(inkwell::module::Linkage::External), // TODO: `Private` emits unrelocated AUIPC?
             None,
         )?;