powerpc/boot: Fix zImage TOC alignment

commit 97ee351b upstream.

Recent toolchains force the TOC to be 256 byte aligned. We need to
enforce this alignment in the zImage linker script, otherwise pointers
to our TOC variables (__toc_start) could be incorrect. If the actual
start of the TOC and __toc_start don't have the same value we crash
early in the zImage wrapper.

Suggested-by: Alan Modra <amodra@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>