Update -ftrivial-auto-var-init flags for clang >= 16

As of clang 16, the -ftrivial-auto-var-init=zero option no longer needs
-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
to enable the option. Only add it for older clang versions.

PR: 271047
Reviewed by: emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D40208