Updating ifdefs to account for xlclang compiler frontend on AIX.

The fallback DEP works fine there. XLC should be unaffected.

CLA: trivial

Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18892)

(cherry picked from commit df1e33bc)