Update -ftrivial-auto-var-init flags for clang >= 16 and gcc >= 12

[dim]

Summary

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.

Also, now that gcc 12 and later supports -ftrivial-auto-var-init, add it
to bsd.compiler.mk's "init-all" feature.

PR: 271047
MFC after: 3 days

[dim]

• Fix typo.

[emaste]

IMO bsd.compiler.mk (enabling for GCC) can be committed separately