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/Cargo.toml

@@ -14,6 +14,9 @@ edition = "2018"
 cranelift-codegen = { path = "../codegen", version = "0.75.0", default-features = false }
 target-lexicon = "0.12"
 
+[target.'cfg(target_arch = "s390x")'.dependencies]
+libc = "0.2.95"
+
 [features]
 default = ["std"]
 std = ["cranelift-codegen/std"]