selftests/powerpc: Run per_event_excludes test on Power8 or later

The per_event_excludes test wants to run on Power8 or later. But
currently it checks that AT_BASE_PLATFORM *equals* power8, which means
it only runs on Power8.

Fix it to check for the ISA 2.07 feature, which will be set on Power8
and later CPUs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200716122142.3776261-1-mpe@ellerman.id.au