kernel: restore crtsavres.o to enable building external modules on powerpc

kernel: restore crtsavres.o to enable building external modules on powerpc

Fixes [YOCTO #1276]

As of Linux kernel version 3.0.1, and much earlier, the make clean target
removes arch/powerpc/lib/crtsavres.o. As this object is present in
KBUILD_LDFLAGS_MODULE, it is required to build external modules, and should
therefor not be removed by make clean.

While I do not advocate fixing buggy kernels in the generic kernel classes,
we should probably account for this one in kernel.bbclass as it affects
such a long list of kernel versions.

(From OE-Core rev: 2e83e6755441cb14bd907d306974338c15173189)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Bruce Ashfield <bruce.ashfield@windriver.com>
CC: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>