When using clang -mcpu=CPUNAME+FEATURELIST,
the implied features defined by CPUNAME are
not obtained, as the entire string is passed.
This fixes that by spiting the cpuname
string in the first +, if any.
For example, when using
clang -### --target=arm-arm-none-eabi -march=armv7-a -mcpu=cortex-a8+nocrc
implied by cortex-a8 is missing.