Add a work-in-progress backend for x86_64 using the new instruction selection;

Most of the work is credited to Julian Seward.

Co-authored-by: Julian Seward <jseward@acm.org>
Co-authored-by: Chris Fallin <cfallin@mozilla.com>

cranelift/codegen/src/isa/x86/mod.rs

@@ -53,12 +53,23 @@ fn isa_constructor(
         PointerWidth::U32 => &enc_tables::LEVEL1_I32[..],
         PointerWidth::U64 => &enc_tables::LEVEL1_I64[..],
     };
-    Box::new(Isa {
-        triple,
-        isa_flags: settings::Flags::new(&shared_flags, builder),
-        shared_flags,
-        cpumode: level1,
-    })
+
+    let isa_flags = settings::Flags::new(&shared_flags, builder);
+
+    if isa_flags.use_new_backend() {
+        #[cfg(not(feature = "x64"))]
+        panic!("new backend x86 support not included by cargo features!");
+
+        #[cfg(feature = "x64")]
+        super::x64::isa_builder(triple).finish(shared_flags)
+    } else {
+        Box::new(Isa {
+            triple,
+            isa_flags,
+            shared_flags,
+            cpumode: level1,
+        })
+    }
 }
 
 impl TargetIsa for Isa {