llext: arc: enforce bitness match

Enforces bitness match of ARC compilers and linkers invoked
building LLEXT apps by inheriting -mcpu flag from Zephyr.
The ARC toolchain becomes confused without the -mcpu flag.
On qemu_hs5x we see GCC select elf32-littlearc64 for
linking an object previously compiled as elf64-littlearc64.

Fixes #80949

Signed-off-by: Lauren Murphy <lauren.murphy@intel.com>