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/isa/x86/mod.rs

@@ -14,7 +14,7 @@ mod legalize;
 mod opcodes;
 mod recipes;
 mod registers;
-mod settings;
+pub(crate) mod settings;
 
 pub(crate) fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
     let settings = settings::define(&shared_defs.settings);