mbed_retarget.cpp: Combine ARMC5 and ARMC6 tests
mbed_retarget.cpp: Combine ARMC5 and ARMC6 tests

A couple of places in mbed_retarget.cpp were testing for either ARMC5 or
ARMC6 in a long-winded fashion. Testing for __ARMCC_VERSION being
defined is sufficient.

mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os