Clean way to patch the std

crates/std/build.rs

@@ -0,0 +1,8 @@
+use std::env;
+
+pub fn main() {
+    println!(
+        "cargo:rustc-env=STD_ENV_ARCH={}",
+        env::var("CARGO_CFG_TARGET_ARCH").unwrap()
+    );
+}