This instruction was introduced on the z14 to accelerate "limbified"
multiplications for certain cryptographic algorithms. This change allows
it to be used in Go assembly.

Change-Id: Ic93dae7fec1756f662874c08a5abc435bce9dd9e
Reviewed-on: https://go-review.googlesource.com/109695
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>