s390x: Add z14 support

* Add support for processor features (including auto-detection).

* Move base architecture set requirement back to z14.

* Add z15 feature sets and re-enable z15-specific code generation
  when required features are available.

cranelift/native/src/lib.rs

@@ -125,6 +125,26 @@ pub fn builder_with_options(
         }
     }
 
+    // There is no is_s390x_feature_detected macro yet, so for now
+    // we use getauxval from the libc crate directly.
+    #[cfg(all(target_arch = "s390x", target_os = "linux"))]
+    {
+        use cranelift_codegen::settings::Configurable;
+
+        if !infer_native_flags {
+            return Ok(isa_builder);
+        }
+
+        let v = unsafe { libc::getauxval(libc::AT_HWCAP) };
+        const HWCAP_S390X_VXRS_EXT2: libc::c_ulong = 32768;
+        if (v & HWCAP_S390X_VXRS_EXT2) != 0 {
+            isa_builder.enable("has_vxrs_ext2").unwrap();
+            // There is no separate HWCAP bit for mie2, so assume
+            // that any machine with vxrs_ext2 also has mie2.
+            isa_builder.enable("has_mie2").unwrap();
+        }
+    }
+
     // squelch warnings about unused mut/variables on some platforms.
     drop(&mut isa_builder);
     drop(infer_native_flags);