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.

crates/jit/src/link.rs

@@ -98,12 +98,13 @@ fn apply_reloc(
             write_unaligned(reloc_address as *mut u32, reloc_delta_u32);
         },
         #[cfg(target_pointer_width = "64")]
-        (RelocationKind::Relative, RelocationEncoding::X86Branch, 32) => unsafe {
+        (RelocationKind::Relative, RelocationEncoding::Generic, 32) => unsafe {
             let reloc_address = body.add(offset as usize) as usize;
             let reloc_addend = r.addend() as isize;
             let reloc_delta_u64 = (target_func_address as u64)
                 .wrapping_sub(reloc_address as u64)
                 .wrapping_add(reloc_addend as u64);
+            // TODO implement far calls mode in x64 new backend.
             assert!(
                 reloc_delta_u64 as isize <= i32::max_value() as isize,
                 "relocation too large to fit in i32"