CMake: Fix Cortex-A9 flags for GCC_ARM toolchain

The -mcpu=cortex-a9 flag conflicts with the march=armv7-a flag.
Opted to keep mcpu=cortex-a9 as it is more specific and
allows GCC to perform better optimization.

The compiler is also changed to use soft-float ABI as it
was necessary to successfully build. Without it the application
appears to be built with soft-float ABI and it conflicts with
the previous setting which was built with hard-float ABI.
This may be related to: https://gitlab.kitware.com/cmake/cmake/-/issues/21173

tools/cmake/cores/Cortex-A9.cmake

@@ -5,9 +5,8 @@ if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
     list(APPEND common_options
         "-mthumb-interwork"
         "-marm"
-        "-march=armv7-a"
         "-mfpu=vfpv3"
-        "-mfloat-abi=hard"
+        "-mfloat-abi=softfp"
         "-mno-unaligned-access"
         "-mcpu=cortex-a9"
     )