kselftest: make callers of ksft_exit_skip() output the reason for skipping

Make the three tests that did use the old ksft_ext_skip()
(breakpoints/breakpoint_test_arm64, breakpoints/step_after_suspend_test,
and membarrier_test) use the new one, with an output for the
reason for skipping all the tests.

Signed-off-by: Paul Elder <paul.elder@pitt.edu>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>