Make settings::Flags::new consume the Builder.

This makes it more clear what the relationship is between the Builder
and the resulting Flags.

lib/codegen/src/isa/arm64/mod.rs

@@ -32,7 +32,7 @@ pub fn isa_builder() -> IsaBuilder {
 
 fn isa_constructor(
     shared_flags: shared_settings::Flags,
-    builder: &shared_settings::Builder,
+    builder: shared_settings::Builder,
 ) -> Box<TargetIsa> {
     Box::new(Isa {
         isa_flags: settings::Flags::new(&shared_flags, builder),