crypto/ecdsa: revert ECDSA assembly on s390x [1.13 backport]

[gopherbot]

@billotosyr requested issue #34927 to be considered for backport to the next 1.13 minor release.

@gopherbot, please consider backport to 1.13.2

[gopherbot]

Change https://golang.org/cl/201317 mentions this issue: crypto/ecdsa: fix buffer size on s390x for ecdsa

[odeke-em]

I don't think we meant to close this without a backport.

[gopherbot]

Change https://golang.org/cl/201337 mentions this issue: [release-branch.go1.13] crypto/ecdsa: fix buffer size on s390x for ecdsa

[gopherbot]

Change https://golang.org/cl/201361 mentions this issue: [release-branch.go1.13] crypto/ecdsa: remove s390x assembly

[FiloSottile]

Fixed by CL 201361