arch/arc: Add -mcpu flag to TOOLCHAIN_LD_FLAGS
arch/arc: Add -mcpu flag to TOOLCHAIN_LD_FLAGS

Make sure the linker knows what the target is, in case it needs to find
additional target-specific libraries (like picolibc).

Signed-off-by: Keith Packard <fd7f967895e9f35e58ec8a62a847a54d7fa7275f@keithp.com>

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