Make -Os a fatal error

As per http://code.google.com/p/nativeclient/issues/detail?id=2284
using -Os produces bad code.  So disallow it until that gets fixed.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=2535
TEST= tested manually

R=khim@google.com,bradchen@google.com

Review URL: http://codereview.chromium.org/9186013
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