fix: harden reassociation barriers for fast-math nearbyint

[DiamonDinoia]

There is a but in nearbyint -fassociative-math breaks it as it does not define __FAST_MATH__.
Also the barrier used was causing a stack spill.

I centralized a barrier function that we can use everywhere in the code and used it in the places I know it helps.

Now we can just use reassociation_barrier to avoid compiler reordering of instructions.

Let me know if you like the internal API and if you need changes to it. I find that this is the solution that minimizes ifdef boilerplate and allows to dispatch to all archs. (With c++17 this will be simpler).

Cheers,
Marco