ghc-7.6: add missing Num instance for Bits

Fixes the following build failure:
    src/runtime/haskell/Data/Binary/IEEE754.lhs:256:17:
        Could not deduce (Num a) arising from a use of `mask'
        from the context (Bits a)
          bound by the type signature for
                     clamp :: Bits a => BitCount -> a -> a

src/runtime/haskell/Data/Binary/IEEE754.lhs

@@ -252,7 +252,7 @@ rawToBytes raw = integerToBytes mashed width where
 in positions above the count.
 
 \begin{code}
-clamp :: Bits a => BitCount -> a -> a
+clamp :: (Num a, Bits a) => BitCount -> a -> a
 clamp = (.&.) . mask where
 	mask 1 = 1
 	mask n | n > 1 = (mask (n - 1) `shiftL` 1) + 1