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.

tools/cmake/cores/Cortex-A.cmake

@@ -8,13 +8,13 @@ if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
         "-mfpu=vfpv3"
         "-mfloat-abi=softfp"
         "-mno-unaligned-access"
-        "-mcpu=${CMAKE_SYSTEM_PROCESSOR}"
+        "-mcpu=${MBED_CPU_CORE}"
     )
 elseif(${MBED_TOOLCHAIN} STREQUAL "ARM")
     list(APPEND common_options
         "-mfpu=vfpv3"
         "-mfloat-abi=hard"
-        "-mcpu=${CMAKE_SYSTEM_PROCESSOR}"
+        "-mcpu=${MBED_CPU_CORE}"
     )
 endif()