x86_64 compiler + elf output (can compile code that returns exit code)

src/backend/mod.rs

@@ -1 +1,18 @@
 pub mod arch;
+pub mod elf;
+mod test;
+pub use arch::*;
+pub use test::*;
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Addr(u64);
+impl Addr {
+    const NONE: Self = Self(!0);
+    pub fn val(&self) -> u64 {
+        self.0
+    }
+}
+
+pub enum Asm {
+    X86_64(x86_64::Asm),
+}