build: use -O2 instead of -Os for ARC with SDK 0.9

This is intended as a temporary fix for ZEP-1882.
Investigation for this bug has shown that the current SDK 0.9 compiler
for ARC generates incorrect code under -Os optimization level.

Kuo-Lang Tseng found out that SDK 0.8.2 does not have this issue, and
that lowering optimization level to -O2 fixes the issue with SDK 0.9.
Juro Bystricky is working on an updated SDK with ARC GCC 6.3.0, but
there are still problems with it, so he also suggested to use -O2 in
the meantime.

Instead of blindly setting -O2 for all toolchains and architectures,
let Makefile.toolchain.zephyr make the decision for ARC and 0.9 only.
Tested with hello_world, CONFIG_ADC=y and BOARD=arduino_101_sss.

Jira: ZEP-1882
Change-Id: Ifde2e3950c9d93eed8982149805acfda9d13a94f
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>