kernel,module-base.bbclass: Improve KERNEL_LD & KERNEL_AR variables

KERNEL_LD was using ${LD} in it's definition, which is not correct for
different ABIs such as x32 or i386 on x86_64 machine. This brings it
into sync with the corresponding gcc settings, likewise the same with
the KERNEL_AR variable.

[RP: Updated commit message]
Signed-off-by: Nitin A Kamble <nitin.a.kamble@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>