[release-branch.go1.12] math/big: fix the bug in assembly implementation of shlVU on arm64

For the case where the addresses of parameter z and x of the function
shlVU overlap and the address of z is greater than x, x (input value)
can be polluted during the calculation when the high words of x are
overlapped with the low words of z (output value).

Updates #31084
Fixes #32940

Change-Id: I9bb0266a1d7856b8faa9a9b1975d6f57dece0479
Reviewed-on: https://go-review.googlesource.com/c/go/+/169780


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit 503e6ccd)
Reviewed-on: https://go-review.googlesource.com/c/go/+/185041
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>