math/big: simplify hasVX checking on s390x

Originally, we use an assembly function that returns a boolean result to
tell whether the machine has vector facility or not. It is now no longer
needed when we can directly use cpu.S390X.HasVX variable.

Change-Id: Ic1dae851982532bcfd9a9453416c112347f21d87
Reviewed-on: https://go-review.googlesource.com/c/go/+/230318


Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>