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/Cargo.toml

@@ -58,10 +58,12 @@ x86 = []
 arm32 = []
 arm64 = []
 riscv = []
+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"