powerpc/mpc5200: Remove VLA usage

powerpc/mpc5200: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
switches to using a stack size large enough for the saved routine and
adds a sanity check making sure the routine doesn't overflow into the
0x600 exception handler.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>