crypto/ecdsa: use FillBytes on s390x

crypto/ecdsa: use FillBytes on s390x

Originally, zeroExtendAndCopy is used to pad src with leading zeros and
copy the padded src into the destination. It is no longer needed after
CL 230397 introduced FillBytes. We can simply use that and remove the
zeroExtendAndCopy function. It is cleaner and reduces some allocation.

In addition, this patch tries to avoid calling hashToInt function in
both Sign and Verify function so some allocation is reduced.

Benchmarks:
name             old alloc/op   new alloc/op   delta
SignP256-8         1.60kB ± 0%    1.49kB ± 0%    -7.23%  (p=0.000 n=20+20)
SignP384-8         1.74kB ± 0%    1.59kB ± 0%    -8.50%  (p=0.000 n=20+18)
VerifyP256-8         176B ± 0%        0B       -100.00%  (p=0.000 n=20+20)
KeyGeneration-8      640B ± 0%      640B ± 0%      ~     (all equal)

name             old allocs/op  new allocs/op  delta
SignP256-8           22.0 ± 0%      17.0 ± 0%   -22.73%  (p=0.000 n=20+20)
SignP384-8           22.0 ± 0%      17.0 ± 0%   -22.73%  (p=0.000 n=20+20)
VerifyP256-8         7.00 ± 0%      0.00       -100.00%  (p=0.000 n=20+20)
KeyGeneration-8      13.0 ± 0%      13.0 ± 0%      ~     (all equal)

Change-Id: Ic4c95191eded55deb3420d97db501689f3b173c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/232297
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>