[release-branch.go1.15] math/big: remove the s390x assembly for shlVU and shrVU

The s390x assembly for shlVU does a forward copy when the shift amount s
is 0. This causes corruption of the result z when z is aliased to the
input x.

This fix removes the s390x assembly for both shlVU and shrVU so the pure
go implementations will be used.

Test cases have been added to the existing TestShiftOverlap test to
cover shift values of 0, 1 and (_W - 1).

Fixes #45335

Change-Id: I75ca0e98f3acfaa6366a26355dcd9dd82499a48b
Reviewed-on: https://go-review.googlesource.com/c/go/+/274442


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
(cherry picked from commit b1369d58)
Reviewed-on: https://go-review.googlesource.com/c/go/+/320169


Trust: Carlos Amedee <carlos@golang.org>
Run-TryBot: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>