[POWERPC] Fix ppc kernels after build-id addition

This patch fixes arch/ppc kernels, at least for prep subarch, after
build-id addition.  Without this, kernels were 3 times the size and
bootloader refused to load them.  Now they are back to normal again.

Tested only with Roland McGrath's "Use LDFLAGS_MODULE only for .ko
links" patch applied - boots and works fine.

Signed-off-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Paul Mackerras <paulus@samba.org>