Openssl fails to compile on Debian with kfreebsd kernels

(kfreebsd-amd64, kfreebsd-i386). The error reported by the compiler
is:

../crypto/uid.c: In function 'OPENSSL_issetugid':
../crypto/uid.c:50:22: error: 'AT_SECURE' undeclared (first use in this function)
   50 |     return getauxval(AT_SECURE) != 0;
      |                      ^~~~~~~~~

This commit changes the code to use the freebsd code in this case.
This fixes the compilation.

CLA: trivial

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16477)