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/mod.rs

@@ -75,15 +75,6 @@ impl<'ctx> Function<'ctx> {
         self.name.as_str()
     }
 
-    /// Checks whether the function is defined outside of the front-end.
-    pub fn is_name_external(name: &str) -> bool {
-        name.starts_with("llvm.")
-            || (name.starts_with("__")
-                && name != self::runtime::FUNCTION_ENTRY
-                && name != self::runtime::FUNCTION_DEPLOY_CODE
-                && name != self::runtime::FUNCTION_RUNTIME_CODE)
-    }
-
     /// Returns the LLVM function declaration.
     pub fn declaration(&self) -> Declaration<'ctx> {
         self.declaration
@@ -223,30 +214,11 @@ impl<'ctx> Function<'ctx> {
         self.stack.get(name).copied()
     }
 
-    /// Removes the pointer to a stack variable.
-    pub fn remove_stack_pointer(&mut self, name: &str) {
-        self.stack.remove(name);
-    }
-
     /// Returns the return entity representation.
     pub fn r#return(&self) -> Return<'ctx> {
         self.r#return
     }
 
-    /// Returns the pointer to the function return value.
-    /// # Panics
-    /// If the pointer has not been set yet.
-    pub fn return_pointer(&self) -> Option<Pointer<'ctx>> {
-        self.r#return.return_pointer()
-    }
-
-    /// Returns the return data size in bytes, based on the default stack alignment.
-    /// # Panics
-    /// If the pointer has not been set yet.
-    pub fn return_data_size(&self) -> usize {
-        self.r#return.return_data_size()
-    }
-
     /// Returns the function entry block.
     pub fn entry_block(&self) -> inkwell::basic_block::BasicBlock<'ctx> {
         self.entry_block