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/Cargo.toml

@@ -66,7 +66,6 @@ x64 = [] # New work-in-progress codegen backend for x86_64 based on the new isel
 # Option to enable all architectures.
 all-arch = [
     "x86",
-    "x64",
     "arm32",
     "arm64",
     "riscv"