ARC: MWDT: use LLVM C++ library instead of MWDT one
ARC: MWDT: use LLVM C++ library instead of MWDT one

The MWDT toolchain provides two C++ libraries. Let's use
LLVM C++ library.

Signed-off-by: Eugeniy Paltsev <ec089b543c2604b626d7ce60c03e4d074ed102c7@synopsys.com>
Signed-off-by: Evgeniy Paltsev <7de4ab8b01334d64e4e84734c688a8a936513edc@gmail.com>

zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr