Enable -ftrivial-auto-var-init flags for gcc >= 12

Now that gcc >= 12 supports -ftrivial-auto-var-init, add it to
bsd.compiler.mk's "init-all" feature.

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

(cherry picked from commit c32736222cdd92e1497d1154d81c9005757fb308)