test/timing_load_creds.c: use OPENSSL_SYS_ macros

test/timing_load_creds.c: use OPENSSL_SYS_ macros

A previous change was only half done.  To avoid such mistakes again, we
switch to using the OPENSSL_SYS_ macros, as the are clearer than having
to check a pile of very platform and compiler specific macros.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/19720)