ARM64 backend, part 8 / 11: integration.

This patch ties together the new backend infrastructure with the
existing Cranelift codegen APIs.

With all patches in this series up to this patch applied, the ARM64
compiler is now functional and can be used. Two uses of this
functionality -- filecheck-based tests and integration into wasmtime --
will come in subsequent patches.

cranelift/codegen/src/verifier/flags.rs

@@ -28,13 +28,17 @@ pub fn verify_flags(
     errors: &mut VerifierErrors,
 ) -> VerifierStepResult<()> {
     let _tt = timing::verify_flags();
-    let mut verifier = FlagsVerifier {
-        func,
-        cfg,
-        encinfo: isa.map(|isa| isa.encoding_info()),
-        livein: SecondaryMap::new(),
-    };
-    verifier.check(errors)
+    if isa.is_none() || isa.unwrap().get_mach_backend().is_none() {
+        let mut verifier = FlagsVerifier {
+            func,
+            cfg,
+            encinfo: isa.map(|isa| isa.encoding_info()),
+            livein: SecondaryMap::new(),
+        };
+        verifier.check(errors)
+    } else {
+        Ok(())
+    }
 }
 
 struct FlagsVerifier<'a> {