selftests/powerpc: Fix emit_tests to work with run_kselftest.sh

In order to use run_kselftest.sh the list of tests must be emitted to
populate kselftest-list.txt.

The powerpc Makefile is written to use EMIT_TESTS. But support for
EMIT_TESTS was dropped in commit d4e59a53 ("selftests: Use runner.sh
for emit targets"). Although prior to that commit a548de0f
("selftests: lib.mk: add test execute bit check to EMIT_TESTS") had
already broken run_kselftest.sh for powerpc due to the executable check
using the wrong path.

It can be fixed by replacing the EMIT_TESTS definitions with actual
emit_tests rules in the powerpc Makefiles. This makes run_kselftest.sh
able to run powerpc tests:

  $ cd linux
  $ export ARCH=powerpc
  $ export CROSS_COMPILE=powerpc64le-linux-gnu-
  $ make headers
  $ make -j -C tools/testing/selftests install
  $ grep -c "^powerpc" tools/testing/selftests/kselftest_install/kselftest-list.txt
  182

Fixes: d4e59a53 ("selftests: Use runner.sh for emit targets")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230921072623.828772-1-mpe@ellerman.id.au