Stocks Sag as Oil Marches Higher  NEW YORK (Reuters) - U.S. stocks tumbled on Thursday as oil  prices marched above \$48 a barrel, but Google Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=GOOG.O target=/stocks/quickinfo/fullquote"&gt;GOOG.O&lt;/A&gt;  provided some excitement after shares of the Internet search  company surged on their debut.