CMake: Fix Cortex-A9 builds with GCC_ARM

Add the `-march` flag for Cortex-A9 GCC_ARM in order to
generate instructions for the armv7-a machine type.

Author: hugueskamba

tools/cmake/cores/Cortex-A9.cmake

@@ -1,10 +1,11 @@
-# Copyright (c) 2020 ARM Limited. All rights reserved.
+# Copyright (c) 2020-2021 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
 if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
     list(APPEND common_options
         "-mthumb-interwork"
         "-marm"
+        "-march=armv7-a"
         "-mfpu=vfpv3"
         "-mfloat-abi=hard"
         "-mno-unaligned-access"