Re-enable sync_with_stdio

Otherwise writing to std::cerr is not thread-safe (in particular,
lines will be randomly duplicated).

src/libmain/shared.cc

@@ -117,9 +117,6 @@ void initNix()
     std::cerr.rdbuf()->pubsetbuf(buf, sizeof(buf));
 #endif
 
-    // FIXME: do we need this? It's not thread-safe.
-    std::ios::sync_with_stdio(false);
-
     if (getEnv("IN_SYSTEMD") == "1")
         logType = ltSystemd;