Add TLS support for ELF and MachO (#1174)

* Add TLS support
* Add binemit and legalize tests
* Spill all caller-saved registers when necessary

cranelift/codegen/meta/src/shared/settings.rs

@@ -131,6 +131,14 @@ pub(crate) fn define() -> SettingGroup {
         false,
     );
 
+    settings.add_enum(
+        "tls_model",
+        r#"
+            Defines the model used to perform TLS accesses.
+        "#,
+        vec!["none", "elf_gd", "macho", "coff"],
+    );
+
     // Settings specific to the `baldrdash` calling convention.
 
     settings.add_enum(