Test for virtual counter readability on ARMv8

The upstream kernel makes the CNTVCT register readable from userspace to
enable fast gettimeofday() implementations.  Test that vendors have not
disabled this capability.

Create a new CpuInstructions test class for this, in anticipation of
other tests which follow the same pattern (check whether SIGILL is
signaled while running some operation).

Bug: 15940803

Change-Id: I4f1878eb68b42ce19e97f9257d4f633de18788f8
Signed-off-by: Greg Hackmann <ghackmann@google.com>