[mlir][flang] add fast math attribute to fcmp (#74315)

`llvm.fcmp` does support fast math attributes therefore so should
`arith.cmpf`.

The heavy churn in flang tests are because flang sets
`fastmath<contract>` by default on all operations that support the fast
math interface. Downstream users of MLIR should not be so effected.

This was requested in https://github.com/llvm/llvm-project/issues/74263