ARC: MWDT: fix SMP build for MWDT toolchain
ARC: MWDT: fix SMP build for MWDT toolchain

Metaware assembler doesn't accept '@' symbol in the beginning
of symbol name like GNU does.

Drop excessive '@' for _curr_cpu symbol.

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

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