fix CMake errors on Cortex-A targets (#293)

Author: wdx04

cmsis/CMSIS_5/CMSIS/RTOS2/RTX/CMakeLists.txt

@@ -32,7 +32,7 @@ if(${CMAKE_CROSSCOMPILING})
     function(_mbed_get_cortex_a_exception_handlers)
         foreach(key ${MBED_TARGET_LABELS})
             if(${key} STREQUAL CORTEX_A)
-                target_sources(mbed-rtos INTERFACE Config/TARGET_CORTEX_A/handlers.c)
+                target_sources(mbed-rtos-sources INTERFACE Config/TARGET_CORTEX_A/handlers.c)
             endif()
         endforeach()
     endfunction()