kern.mk: quiet bfd executable stack warnings

[rlibby]

Summary

Also add -Wtrampolines for GCC. Clang already disallows nested
functions, -Wtrampolines just provides an equivalent warning/error for
GCC and documents that we aren't just papering over warnings for code
that really does use executable stacks.

Test Plan

env CROSS_TOOLCHAIN=amd64-gcc14 make buildkernel

[kib]

So what are the sources that cause the warning? It would be better to fix them instead, but I do not object against this somewhat large hammer.

[rlibby]

$ curl https://ci.freebsd.org/job/FreeBSD-main-amd64-gcc14_build/3606/consoleText -o /tmp/FreeBSD-main-amd64-gcc14_build.3606.consoleText
$ grep "executable stack" /tmp/FreeBSD-main-amd64-gcc14_build.3606.consoleText > /tmp/grep-executable-stack.log
$ wc -l /tmp/grep-executable-stack.log 
72 /tmp/grep-executable-stack.log

grep-executable-stack.log9 KBDownload

It appears many but not all may be pre-assembled objects or hand-written assembly.

We could try to pick at these individually instead. This is obviously not urgent, I was just trying to clean up some warnings.

[rlibby]

Abandoning this for now, though I still think it would be fine to take. I addressed some of the underlying issues with D56894 and D56946.

The warnings that are left here are mostly coming from contrib binary blobs, but also some contrib assembly:

grep-executable-stack.3771.log7 KBDownload

The warning is no longer currently issued for the kernel itself, but for various module builds.