kern/60839: use -ftrivial-auto-var-init

["campbell+netbsd%mumble.net@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60839
>Category:       kern
>Synopsis:       use -ftrivial-auto-var-init
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Fri Oct 02 18:10:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current
>Organization:
Acme Variable Initialization, Ink.
>Environment:
>Description:

	As proposed on tech-kern:

	> tl;dr: Turn on gcc/clang compiler option in kernel to fully
	> initialize all kernel stack variables, as zero on release
	> builds and nonzero pattern in current builds, to
	> systematically plug kernel stack memory disclosure leaks with
	> almost zero performance impact?

	https://mail-index.NetBSD.org/tech-kern/2026/07/20/msg031158.html

	Would be nice if the choice of zero vs pattern could easily
	depend on `options DIAGNOSTIC' or `options DEBUG'.

>How-To-Repeat:

>Fix: