-ftrivial-auto-var-init in userland?

I propose to enable -ftrivial-auto-var-init for all libraries and
utilities in userland by default (under gcc and clang), and
specifically to zero-initialize local variables in release builds and
to nonzero-initialize them in current builds.

See https://mail-index.netbsd.org/tech-kern/2026/07/20/msg031158.html
for background.  If there's any userland-specific considerations to
discuss, we can discuss in this thread here.

Thoughts?