machinst x64: Only use the feature flag to enable the x64 new backend;

Before this patch, running the x64 new backend would require both
compiling with --features experimental_x64 and running with
`use_new_backend`.

This patches changes this behavior so that the runtime flag is not
needed anymore: using the feature flag will enforce usage of the new
backend everywhere, making using and testing it much simpler:

    cargo run --features experimental_x64 ;; other CLI options/flags

This also gives a hint at what the meta language generation would look
like after switching to the new backend.

Compiling only with the x64 codegen flag gives a nice compile time speedup.

cranelift/codegen/meta/src/gen_legalizer.rs

@@ -700,6 +700,7 @@ fn gen_isa(
 pub(crate) fn generate(
     isas: &[TargetIsa],
     transform_groups: &TransformGroups,
+    extra_legalization_groups: &[&'static str],
     filename_prefix: &str,
     out_dir: &str,
 ) -> Result<(), error::Error> {
@@ -711,8 +712,14 @@ pub(crate) fn generate(
         fmt.update_file(format!("{}-{}.rs", filename_prefix, isa.name), out_dir)?;
     }
 
+    // Add extra legalization groups that were explicitly requested.
+    for group in extra_legalization_groups {
+        shared_group_names.insert(group);
+    }
+
     // Generate shared legalize groups.
     let mut fmt = Formatter::new();
+    // Generate shared legalize groups.
     let mut type_sets = UniqueTable::new();
     let mut sorted_shared_group_names = Vec::from_iter(shared_group_names);
     sorted_shared_group_names.sort();