A paranoid go at constant time implementation of P256 curve.

This code relies on z13 SIMD instruction set. For zEC12 and below,
the fallback is the existing P256 implementation. To facilitate this
fallback mode, I've refactored the code so that implementations can
be picked at run-time.

Its 'slightly' difficult to grok, but there is ASCII art..

name            old time/op  new time/op  delta
BaseMultP256     419µs ± 3%    27µs ± 1%  -93.65% (p=0.000 n=10+8)
ScalarMultP256  1.05ms ±10%  0.09ms ± 1%  -90.94% (p=0.000 n=10+8)

Change-Id: Ic1ded898a2ceab055b1c69570c03179c4b85b177
Reviewed-on: https://go-review.googlesource.com/31231
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>