CMake: Cortex-A: Use MBED_CPU_CORE to set -mcpu
CMake: Cortex-A: Use MBED_CPU_CORE to set -mcpu

As we're moving away from the use of CMAKE_SYSTEM_PROCESSOR, use
MBED_CPU_CORE instead. They differ in cases, but both GCC and Arm
toolchains support case-insensitive CPU names.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed