linux: arc: disable kernel unwinding to fix modules loading

linux: arc: disable kernel unwinding to fix modules loading

With update of binutils for ARC (this is now based on upstream 2.26)
we noticed issues with loadable kernel modules.

Something like that was happening:
--------------------->8-------------------
mbcache: unknown relocation: 49
insmod: can't insert './mbcache.ko': invalid module format
--------------------->8-------------------

More details could be found in that discussion in binutils mailing list:
http://thread.gmane.org/gmane.comp.gnu.binutils/74662

As of now the simplest work-around is to disable in-kernel unwinder
for now. That will at least allow us to use modules again.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: John Crispin <john@phrozen.org>