Fix fn_sig_for_fn_abi and the coroutine transform for generators

There were three issues previously:
* The self argument was pinned, despite Iterator::next taking an
  unpinned mutable reference.
* A resume argument was passed, despite Iterator::next not having one.
* The return value was CoroutineState<Item, ()> rather than Option<Item>

While these things just so happened to work with the LLVM backend,
cg_clif does much stricter checks when trying to assign a value to a
place. In addition it can't handle the mismatch between the amount of
arguments specified by the FnAbi and the FnSig.

compiler/rustc_codegen_cranelift/build_system/tests.rs

@@ -100,6 +100,15 @@ const BASE_SYSROOT_SUITE: &[TestCase] = &[
     TestCase::build_bin_and_run("aot.issue-72793", "example/issue-72793.rs", &[]),
     TestCase::build_bin("aot.issue-59326", "example/issue-59326.rs"),
     TestCase::build_bin_and_run("aot.neon", "example/neon.rs", &[]),
+    TestCase::custom("aot.gen_block_iterate", &|runner| {
+        runner.run_rustc([
+            "example/gen_block_iterate.rs",
+            "--edition",
+            "2024",
+            "-Zunstable-options",
+        ]);
+        runner.run_out_command("gen_block_iterate", &[]);
+    }),
 ];
 
 pub(crate) static RAND_REPO: GitRepo = GitRepo::github(