crypto/uid.c: use own macro as guard rather than AT_SECURE

It turns out that AT_SECURE may be defined through other means than
our inclusion of sys/auxv.h, so to be on the safe side, we define our
own guard and use that to determine if getauxval() should be used or
not.

Fixes #7932

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7933)

(cherry picked from commit aefb980c)