stackinit: Keep selftest union size small on m68k

The stack frame on m68k is very sensitive to the size of what needs to
be stored. Like done for long string testing, reduce the size of the
large trailing struct in the union initialization testing.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/all/CAMuHMdXW8VbtOAixO7w+aDOG70aZtZ50j1Ybcr8B3eYnRUcrcA@mail.gmail.com
Fixes: e71a29db ("stackinit: Add union initialization to selftests")
Link: https://lore.kernel.org/r/20250204174509.work.711-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>