s390x: Fix vector facility (vx) check in test suite

s390x: Fix vector facility (vx) check in test suite

When checking the prereqisuites of running a vector test case, it is not
sufficient to check for the appropriate CPU facility bit.  It must also be
verified that the kernel and hypervisor(s) have actually enabled the
vector facility.  There are various ways of checking this.  E.g., we could
try executing a vector instruction and handle any signals.  Or we can
check the HWCAP for the appropriate bit.  This patch does the latter.