We already do this for floor/ceil, but RoundToEven was added later.
Intrinsify it also.

name           old time/op  new time/op  delta
RoundToEven-8  3.00ns ± 1%  0.68ns ± 2%  -77.34%  (p=0.000 n=10+10)

Change-Id: Ib158cbceb436c6725b2d9353a526c5c4be19bcad
Reviewed-on: https://go-review.googlesource.com/74852
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>