Add a work-in-progress backend for x86_64 using the new instruction selection;

Most of the work is credited to Julian Seward.

Co-authored-by: Julian Seward <jseward@acm.org>
Co-authored-by: Chris Fallin <cfallin@mozilla.com>

cranelift/codegen/src/isa/mod.rs

@@ -77,6 +77,9 @@ mod riscv;
 #[cfg(feature = "x86")]
 mod x86;
 
+#[cfg(feature = "x64")]
+mod x64;
+
 #[cfg(feature = "arm32")]
 mod arm32;