selftests/powerpc: Skip vmx/vsx/tar/etc tests on older CPUs

Some of our tests use VSX or newer VMX instructions, so need to be
skipped on older CPUs to avoid SIGILL'ing.

Similarly TAR was added in v2.07, and the PMU event used in the stcx
fail test only works on Power8 or later.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200803020719.96114-1-mpe@ellerman.id.au