selftests/powerpc: Add test of mitigation patching

We recently discovered some of our mitigation patching was not safe
against other CPUs running concurrently.

Add a test which enable/disables all mitigations in a tight loop while
also running some stress load. On an unpatched system this almost always
leads to an oops and panic/reboot, but we also check if the kernel
becomes tainted in case we have a non-fatal oops.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210507064225.1556312-1-mpe@ellerman.id.au