LU-1337 build: add missing include dir

LU-1337 build: add missing include dir

arch/include/generated is added since kernel version v3.2.
while at it, also change "uname -m" in .m4 to $target_cpu,
because users may cross building.

Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: Ib9aa526a9318e5c93b05cceba9fc8cdfbf9fef0a
Reviewed-on: http://review.whamcloud.com/3745
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>