Clang do not support -mfix-gr712rc, -mfix-ut700 and -mfix-ut699.
Therefore we adjust the CPU_CFLAGS to remove these when compiling with
Clang. Instead use
GR712RC: -mcpu=gr712rc
GR740: -mcpu=gr740
UT699/UT700: Not currently supported by clang, use LEON3 BSP.