LU-8117 build: fix lustre-ppc64 build on el7

Explicitly clear the LDFLAGS for kernel modules build so that
the vendor's overzealous linker flags don't create build failures.

Test-Parameters: trivial

Change-Id: I427cffb229a19a8ff0c2ba0c3ed2ac135f21e24d
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: http://review.whamcloud.com/20404


Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>