selftests/powerpc: Don't ignore errors from sub Makefiles

Currently we ignore errors from our sub Makefiles. We inherited that
from the top-level selftests Makefile which aims to build and run as
many tests as possible and damn the torpedoes.

For the powerpc tests we'd instead like any errors to fail the build, so
we can automatically catch build failures.

We can achieve the best of both worlds by using -k, which tells make to
keep building when it hits an error, but still reports the error.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>