crypto/elliptic: make P-521 scalar multiplication constant time

Like for P-224, we do the constant time selects to hide the
point-at-infinity special cases of addition, but not the P = Q case,
which presumably doesn't happen in normal operations.

Runtime increases by about 50%, as expected, since on average we were
able to skip half the additions, and the additions reasonably amounted
to half the runtime. Still, the Fiat code is so much faster than big.Int
that we're still more than three time faster overall than pre-CL 315271.

name                   old time/op    new time/op    delta
pkg:crypto/elliptic goos:darwin goarch:arm64
ScalarBaseMult/P521-8    4.18ms ± 3%    1.35ms ± 1%  -67.64%  (p=0.000 n=10+10)
ScalarMult/P521-8        4.17ms ± 2%    1.36ms ± 1%  -67.45%  (p=0.000 n=10+10)
pkg:crypto/ecdsa goos:darwin goarch:arm64
Sign/P521-8              4.23ms ± 1%    1.44ms ± 1%  -66.02%  (p=0.000 n=9+10)
Verify/P521-8            8.31ms ± 2%    2.73ms ± 2%  -67.08%  (p=0.000 n=9+9)
GenerateKey/P521-8       4.15ms ± 2%    1.35ms ± 2%  -67.41%  (p=0.000 n=10+10)

Updates #40171

Change-Id: I782f2b7f33dd60af9b3b75e46d920d4cb47f719f
Reviewed-on: https://go-review.googlesource.com/c/go/+/315274
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

0 files changed, 0 insertions, 0 deletions