Only enable ARMC6 for a few targets
Only enable ARMC6 for a few targets

The affected targets are Renesas targets, USI_WM_BN_BM_22 based
targets, and the MTB_MXCHIP_EMW3166.

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