Use the FIDBR instruction to round floating-point numbers to integers.

name   old time/op  new time/op  delta
Ceil   14.1ns ± 0%   3.0ns ± 0%  -78.89%  (p=0.000 n=10+10)
Floor  6.42ns ± 0%  3.03ns ± 0%  -52.80%  (p=0.000 n=10+10)
Trunc  6.67ns ± 0%  3.03ns ± 0%  -54.57%   (p=0.000 n=10+9)

Change-Id: I3b416f6d0bccaaa9b547de86356471365862399c
Reviewed-on: https://go-review.googlesource.com/27827Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>