arm: Add missing no-ctfconvert for fw_stub.awk target

This target produces a C file not an object file, so using ctfconvert on
it should not be attempted. This keeps it in sync with all other uses of
fw_stub.awk, squashes a warning seen during the build of TEGRA124 on
FreeBSD and avoids the same issue failing the build on non-FreeBSD (such
errors are #ifdef'ed into being warnings on FreeBSD in ctfconvert, which
should be revisited in the future).

Reviewed by: manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41542