add crate for custom isa extensions

Signed-off-by: xermicus <cyrill@parity.io>

crates/extensions/build.rs

@@ -0,0 +1,40 @@
+use std::{env, fs, path::Path, process::Command};
+
+fn compile(source_path: &str, output_path: &str) {
+    let output = Command::new("llc")
+        .args([
+            "-O3",
+            "-filetype=asm",
+            "-mattr=+zbb,+e",
+            source_path,
+            "-o",
+            output_path,
+        ])
+        .output()
+        .expect("should be able to invoke llc");
+
+    assert!(
+        output.status.success(),
+        "failed to compile {}: {:?}",
+        source_path,
+        output
+    );
+}
+
+fn main() {
+    let in_file = "bswap.ll";
+    let out_file = "bswap.s";
+    let out_dir = env::var_os("OUT_DIR").expect("env should have $OUT_DIR");
+    let out_path = Path::new(&out_dir).join(out_file);
+    compile(
+        in_file,
+        out_path.to_str().expect("$OUT_DIR should be UTF-8"),
+    );
+
+    let src_path = Path::new(&out_dir).join("bswap.rs");
+    let src = format!("pub static ASSEMBLY: &str = include_str!(\"{out_file}\");");
+    fs::write(src_path, src).expect("should be able to write in $OUT_DIR");
+
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=bswap.ll");
+}