cmd/compile: intrinsify math.RoundToEven on s390x

cmd/compile: intrinsify math.RoundToEven on s390x

The new RoundToEven function can be implemented as a single FIDBR
instruction on s390x.

name         old time/op  new time/op  delta
RoundToEven  5.32ns ± 1%  0.86ns ± 1%  -83.86%  (p=0.000 n=10+10)

Change-Id: Iaf597e57a0d1085961701e3c75ff4f6f6dcebb5f
Reviewed-on: https://go-review.googlesource.com/74350
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>