FPU: Don't use mask generator for rounding

FPU: Don't use mask generator for rounding

Instead of using the mask generator in the rounding process, this uses
simpler logic to add in a 1 at the appropriate position (bit 2 or bit
31, depending on precision) and mask off the low-order bits.  Since
there are only two positions at which the masking and incrementing
need to be done, we don't need the full generality of the mask
generator.  This reduces the amount of logic and improves timing.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>