kselftest: Add a ksft_perror() helper

The standard library perror() function provides a convenient way to print
an error message based on the current errno but this doesn't play nicely
with KTAP output. Provide a helper which does an equivalent thing in a KTAP
compatible format.

nolibc doesn't have a strerror() and adding the table of strings required
doesn't seem like a good fit for what it's trying to do so when we're using
that only print the errno.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>