selftests/powerpc: Use CLEAN macro to fix make warning

The CLEAN macro was added in 337f1e36 to prevent the

    Makefile:50: warning: overriding recipe for target 'clean'
    ../../lib.mk:124: warning: ignoring old recipe for target 'clean'

style warnings. Expand it's use to fix another case of redefining a
target directly.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230228000709.124727-2-bgray@linux.ibm.com