This CL adds tests of fused multiplication-accumulation
on arm/arm64.

Change-Id: Ic85d5277c0d6acb7e1e723653372dfaf96824a39
Reviewed-on: https://go-review.googlesource.com/c/141652
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>