Use retarget defines

TESTS/filesystem/fopen/fopen.cpp

@@ -40,14 +40,6 @@
  * consistent values for all toolchains */
 #include "platform/mbed_retarget.h"
 
-/* This is needed for stat() test, but is not available on ARMCC.
- * The following checks whether GCC_ARM compiler is being used because:
- * - both the ARMCC compiler and the GCC_ARM compile define __GNUC__.
- * - only the ARMCC compiler defines __ARMCC_VERSION.
- * - hence if __ARMCC_VERSION is not defined and __GNUC__ is defined, it must be GCC_ARM. */
-#if ! defined(__ARMCC_VERSION) && defined(__GNUC__)
-#include <sys/stat.h>
-#endif
 using namespace utest::v1;
 
 /// @cond FSFAT_DOXYGEN_DISABLE