[POWERPC] spufs: enable SPU coredump for kernel-builtin spufs

spu_coredump_calls.owner is NULL in case of a builtin spufs,
so the checks in here break.
Check for the availability of the spu_coredump_calls variable
instead.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>