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.