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/mod.rs

@@ -121,7 +121,11 @@ pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
     match triple.architecture {
         Architecture::Riscv32 | Architecture::Riscv64 => isa_builder!(riscv, "riscv", triple),
         Architecture::I386 | Architecture::I586 | Architecture::I686 | Architecture::X86_64 => {
-            isa_builder!(x86, "x86", triple)
+            if cfg!(feature = "x64") {
+                isa_builder!(x64, "x64", triple)
+            } else {
+                isa_builder!(x86, "x86", triple)
+            }
         }
         Architecture::Arm { .. } => isa_builder!(arm32, "arm32", triple),
         Architecture::Aarch64 { .. } => isa_builder!(aarch64, "arm64", triple),