selftests: exec: return Kselftest Skip code for skipped tests

When execveat test is skipped because of unmet dependencies and/or
unsupported configuration, it exits with error which is treated as
a fail by the Kselftest framework. This leads to false negative
result even when the test could not be run.

Change it to return kselftest skip code when a test gets skipped to
clearly report that the test could not be run.

Change it to use ksft_exit_skip() when kernel doesn't support execveat.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>