selftests: Add install support for the powerpc tests

The bulk of the selftests are actually below the powerpc sub directory.

This adds support for installing them, when on a powerpc machine, or if
ARCH and CROSS_COMPILE are set appropriately.

This is a little more complicated because of the sub directory structure
under powerpc, but much of the common logic in lib.mk is still used. The
net effect of the patch is still a reduction in code.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>