Make improvements

- Fix unused local variable errors
- Remove yoinks from sigaction() header
- Add nox87 and aarch64 to github actions
- Fix cosmocc -fportcosmo in linking mode
- It's now possible to build `make m=llvm o/llvm/libc`

libc/thread/spawn.c

@@ -69,7 +69,7 @@ static int Spawner(void *arg, int tid) {
   rc = spawner->fun(spawner->arg, tid);
   _pthread_ungarbage();
   free(spawner);
-  return 0;
+  return rc;
 }
 
 /**