selftests/powerpc: Check for VSX preservation across userspace preemption

Ensure the kernel correctly switches VSX registers correctly. VSX
registers are all volatile, and despite the kernel preserving VSX
across syscalls, it doesn't have to. Test that during interrupts and
timeslices ending the VSX regs remain the same.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>