powerpc: Use the new post-link pass to check relocations

Currently powerpc has to introduce a dependency on its default build
target zImage in order to run a relocation check pass over the linked
vmlinux. This is deficient because the check is not run if the plain
vmlinux target is built, or if one of the other boot targets is built.

Switch to using the kbuild post-link pass, added in commit fbe6e37d
("kbuild: add arch specific post-link Makefile") in order to run this
check. In future powerpc will use this to do more complicated operations,
but initially using it for something simple is a good first step.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>