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/legalizer/mod.rs

@@ -19,10 +19,24 @@ use crate::flowgraph::ControlFlowGraph;
 use crate::ir::types::{I32, I64};
 use crate::ir::{self, InstBuilder, MemFlags};
 use crate::isa::TargetIsa;
+
+#[cfg(any(
+    feature = "x86",
+    feature = "arm32",
+    feature = "arm64",
+    feature = "riscv"
+))]
 use crate::predicates;
+#[cfg(any(
+    feature = "x86",
+    feature = "arm32",
+    feature = "arm64",
+    feature = "riscv"
+))]
+use alloc::vec::Vec;
+
 use crate::timing;
 use alloc::collections::BTreeSet;
-use alloc::vec::Vec;
 
 mod boundary;
 mod call;