cranelift: Don't build peepmatic-based optimizations in build.rs

Instead, when the `rebuild-peephole-optimizers` feature is enabled, rebuild them
the first time they are used. This allows peepmatic to run when Cranelift's
`Opcode` is defined and available, which paves the way forward for:

* merging `peepmatic_runtime::operator::Operator` and Cranelift's `Opcode` (we
  are wasting a bunch of cycles converting between the two of them), and

* supporting vcode optimizations in `peepmatic`.

cranelift/codegen/build.rs

@@ -90,20 +90,11 @@ fn main() {
     }
 
     #[cfg(feature = "rebuild-peephole-optimizers")]
-    rebuild_peephole_optimizers();
-}
-
-#[cfg(feature = "rebuild-peephole-optimizers")]
-fn rebuild_peephole_optimizers() {
-    use std::path::Path;
-
-    let source_path = Path::new("src").join("preopt.peepmatic");
-    println!("cargo:rerun-if-changed={}", source_path.display());
-
-    let preopt =
-        peepmatic::compile_file(&source_path).expect("failed to compile `src/preopt.peepmatic`");
-
-    preopt
-        .serialize_to_file(&Path::new("src").join("preopt.serialized"))
-        .expect("failed to serialize peephole optimizer to `src/preopt.serialized`");
+    {
+        std::fs::write(
+            std::path::Path::new(&out_dir).join("CRANELIFT_CODEGEN_PATH"),
+            cur_dir.to_str().unwrap(),
+        )
+        .unwrap()
+    }
 }