hash/crc32: 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. This CL
also removes the last occurence of hasVectorFacility function on s390x.

Change-Id: Id20cb746c21eacac5e13344b362e2d87adfe4317
Reviewed-on: https://go-review.googlesource.com/c/go/+/230337


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