FIDBR and FIEBR can be used for floating-point to integer rounding.
The relevant functions (Ceil, Floor and Trunc) will be updated
in a future CL.

Change-Id: I5952d67ab29d5ef8923ff1143e17a8d30169d692
Reviewed-on: https://go-review.googlesource.com/27826Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>