selftests/powerpc: Import Anton's context_switch2 benchmark

This gets referred to a lot in commit messages, so let's pull it into
the selftests.

Almost vanilla from: http://ozlabs.org/~anton/junkcode/context_switch2.c

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Anton Blanchard <anton@samba.org>