cpu/arm/k60: Add correct AROPTS for llvm-ar.

This fixes a build error when using LLVM.

llvm-ar does not support the f modifier (truncate filenames) which is
set by default in the top level Makefile.include.

It is not clear why the f flag is set in the default options or why it
is considered by the Contiki developers that it is necessary to set
their own AROPTS instead of using the default ("rv").

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>