selftests: Add infrastructure for powerpc selftests

This commit adds a powerpc subdirectory to tools/testing/selftests,
for tests that are powerpc specific.

On other architectures nothing is built. The makefile supports cross
compilation if the user sets ARCH and CROSS_COMPILE.

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