selftests/powerpc: Create a new SKIP_IF macro

This patch creates a new macro that skips a test and prints a message to
stderr. This is useful to give an idea why the tests is being skipped,
other than just skipping the test blindly.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>