Suppressed axioms Float.eq_zero_{true,false}, since the latter is

wrong because of +0.0 / -0.0.  
Adapted Clight semantics accordingly.  (Truth value of a float is
defined by comparison Float.cmp Ceq with 0.0, no longer by structural
equality.)


git-svn-id: https://yquem.inria.fr/compcert/svn/compcert/trunk@1342 fca1b0fc-160b-0410-b1d3-a4f43f01ea2e