define poly const in polyval pmull

[tcoratger]

poly can be defined as a constant at a file level and it doesn't need to be recalculated inside the function as it is an hardcoded value.

[newpavlov]

Note that the compiler is "smart enough" to perform constant folding, so this change should have absolutely no runtime effect with enabled optimizations. But it results in a nicer code, so it's worth to merge it.