Floating point rounding modes: we should restrict them slightly

[#ponce]

Why not imposing a rounding mode (ex :  toward minus infinity) ?
And then disallowing to change it in assembly.

I mean, you can still do :
  round(x) as floor(x + 0.5L)
  ceil(x) as floor(x + 1.0L)

Iit would cost some precision with large floating-point number, but not much since rounding such large numbers has few meaning.

It would clear up the rounding-mode mess a bit.