Add TLS support for ELF and MachO (#1174)

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

cranelift/module/src/backend.rs

@@ -72,6 +72,7 @@ where
         name: &str,
         linkage: Linkage,
         writable: bool,
+        tls: bool,
         align: Option<u8>,
     );
 
@@ -107,6 +108,7 @@ where
         id: DataId,
         name: &str,
         writable: bool,
+        tls: bool,
         align: Option<u8>,
         data_ctx: &DataContext,
         namespace: &ModuleNamespace<Self>,
@@ -188,5 +190,7 @@ pub fn default_libcall_names() -> Box<dyn Fn(ir::LibCall) -> String> {
         ir::LibCall::Memcpy => "memcpy".to_owned(),
         ir::LibCall::Memset => "memset".to_owned(),
         ir::LibCall::Memmove => "memmove".to_owned(),
+
+        ir::LibCall::ElfTlsGetAddr => "__tls_get_addr".to_owned(),
     })
 }