selftests/powerpc: Add cache_shape sniff test

This is a very basic test of the new cache shape AUXV entries. All it
does at the moment is look for the entries and error out if we don't
find all the ones we expect. Primarily intended for folks bringing up a
new chip to check that the cache info is making it all the way to
userspace correctly.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>