Add "fabs" (floating-point absolute value) as a unary operator in

Clight and C#minor.  Recognize __builtin_fabs and turn it into this operator.


git-svn-id: https://yquem.inria.fr/compcert/svn/compcert/trunk@1329 fca1b0fc-160b-0410-b1d3-a4f43f01ea2e