selftests: amd-pstate: fix TEST_FILES

Bring back the Python scripts that were initially added with
TEST_GEN_FILES but now with TEST_FILES to avoid having them deleted
when doing a clean.  Also fix the way the architecture is being
determined as they should also be installed when ARCH=x86_64 is
provided explicitly.  Then also append extra files to TEST_FILES and
TEST_PROGS with += so they don't get discarded.

Fixes: ba2d788a ("selftests: amd-pstate: Trigger tbench benchmark and test cpus")
Fixes: a49fb721 ("selftests: amd-pstate: Don't delete source files via Makefile")
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>