selftests/powerpc: Add missing PMU selftests to .gitignores

Some recently added selftests don't have their binaries in .gitignores,
so add them.

I also alphabetically sorted sampling_tests/.gitignore while I was in
there.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220812071632.56095-1-ruscur@russell.cc