armclang: Fix setting CMAKE_SYSTEM_PROCESSOR for mps2_an521

On mps2_an521 GCC_M_CPU gets set to cortex-m33+nodsp, however that is
not a valid setting for CMAKE_SYSTEM_PROCESSOR.  We need to strip the
extra options (+<OPTION>) from GCC_M_CPU so that we get just cortex-m33.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>