internal/cpu: change s390x API to match x/sys/cpu

This CL changes the internal/cpu API to more closely match the
public version in x/sys/cpu (added in CL 163003). This will make it
easier to update the dependencies of vendored code. The most prominent
renaming is from VE1 to VXE for the vector-enhancements facility 1.
VXE is the mnemonic used for this facility in the HWCAP vector.

Change-Id: I922d6c8bb287900a4bd7af70567e22eac567b5c1
Reviewed-on: https://go-review.googlesource.com/c/164437


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