tools/testing/selftests/powerpc: remove redundant CFLAGS in Makefile: "-Wall...

tools/testing/selftests/powerpc: remove redundant CFLAGS in Makefile: "-Wall -O2 -Wall" -> "-O2 -Wall"

There are repeated compiler flags "-Wall" in the Makefile.

The redundant compiler flag appears to have been there from the
 introduction of the Makefile.

Signed-off-by: Chijun Sima <simachijun@qq.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>