s390x: Add ISLE support

This adds ISLE support for the s390x back-end and moves lowering
of most instructions to ISLE.  The only instructions still remaining
are calls, returns, traps, and branches, most of which will need
additional support in common code.

Generated code is not intended to be (significantly) differnet
than before; any additional optimizations due to the ISLE layer
can be added in follow-on patches.  There were a few differences
in some filetests, but those are all just simple register allocation
changes (and all to the better!).

cranelift/codegen/build.rs

@@ -220,6 +220,8 @@ fn get_isle_compilations(crate_dir: &std::path::Path) -> Result<IsleCompilations
         make_isle_source_path_relative(&cur_dir, crate_dir.join("src").join("isa").join("x64"));
     let src_isa_aarch64 =
         make_isle_source_path_relative(&cur_dir, crate_dir.join("src").join("isa").join("aarch64"));
+    let src_isa_s390x =
+        make_isle_source_path_relative(&cur_dir, crate_dir.join("src").join("isa").join("s390x"));
 
     // This is a set of ISLE compilation units.
     //
@@ -258,6 +260,19 @@ fn get_isle_compilations(crate_dir: &std::path::Path) -> Result<IsleCompilations
                     src_isa_aarch64.join("lower.isle"),
                 ],
             },
+            // The s390x instruction selector.
+            IsleCompilation {
+                output: src_isa_s390x
+                    .join("lower")
+                    .join("isle")
+                    .join("generated_code.rs"),
+                inputs: vec![
+                    clif_isle.clone(),
+                    prelude_isle.clone(),
+                    src_isa_s390x.join("inst.isle"),
+                    src_isa_s390x.join("lower.isle"),
+                ],
+            },
         ],
     })
 }