crypto/elliptic: utilize faster z14 multiply/square instructions (when available)

In the s390x assembly implementation of NIST P-256 curve, utilize faster multiply/square
instructions introduced in the z14. These new instructions are designed for crypto
and are constant time. The algorithm is unchanged except for faster
multiplication when run on a z14 or later. On z13, the original mutiplication
(also constant time) is used.

P-256 performance is critical in many applications, such as Blockchain.

name            old time      new time     delta
BaseMultP256    24396 ns/op   21564 ns/op  1.13x
ScalarMultP256  87546 ns/op   72813 ns/op. 1.20x

Change-Id: I7e6d8b420fac56d5f9cc13c9423e2080df854bac
Reviewed-on: https://go-review.googlesource.com/c/146022


Reviewed-by: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Michael Munday <mike.munday@ibm.com>