Cranelift: Add LibCall::Memcmp

The comment says the enum is "likely to grow" and the function's been in libc since C89, so hopefully this is ok.

I'd like to use it for emitting things like array equality.

cranelift/module/src/lib.rs

@@ -73,6 +73,7 @@ pub fn default_libcall_names() -> Box<dyn Fn(ir::LibCall) -> String + Send + Syn
         ir::LibCall::Memcpy => "memcpy".to_owned(),
         ir::LibCall::Memset => "memset".to_owned(),
         ir::LibCall::Memmove => "memmove".to_owned(),
+        ir::LibCall::Memcmp => "memcmp".to_owned(),
 
         ir::LibCall::ElfTlsGetAddr => "__tls_get_addr".to_owned(),
     })