selftests/powerpc: Don't touch VMX/VSX on older CPUs

If we're running on a CPU without VMX/VSX then don't touch them. This
is fragile, the compiler could spill a VMX/VSX register and break the
test anyway. But in practice it seems to work, ie. the test runs to
completion on a system without VSX with this change.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819015727.1977134-8-mpe@ellerman.id.au