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/mod.rs

@@ -6,10 +6,10 @@ use std::fmt;
 mod arm32;
 mod arm64;
 mod riscv;
-mod x86;
+pub(crate) mod x86;
 
 /// Represents known ISA target.
-#[derive(Copy, Clone)]
+#[derive(PartialEq, Copy, Clone)]
 pub enum Isa {
     Riscv,
     X86,