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/src/isa/x64/settings.rs

@@ -0,0 +1,9 @@
+//! x86 Settings.
+
+use crate::settings::{self, detail, Builder};
+use core::fmt;
+
+// Include code generated by `cranelift-codegen/meta/src/gen_settings.rs:`. This file contains a
+// public `Flags` struct with an impl for all of the settings defined in
+// `cranelift-codegen/meta/src/isa/x86/settings.rs`.
+include!(concat!(env!("OUT_DIR"), "/settings-x86.rs"));