Generate a compiler error when ARMCC is not using GNU extentions

The GNU extensions enableable via the "--gnu" compile option, is
mandatory for the ARMCC to benefit from GNU atomic routines and macros.
This commit adds compilation error to notify user about ARMCC

Signed-off-by: Haithem Rahmani <haithem.rahmani@st.com>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>

lib/compiler.h

@@ -16,6 +16,8 @@
 # include <metal/compiler/gcc/compiler.h>
 #elif defined(__ICCARM__)
 # include <metal/compiler/iar/compiler.h>
+#elif defined(__CC_ARM)
+# error "MDK-ARM ARMCC compiler requires the GNU extentions to work correctly"
 #else
 # error "Missing compiler support"
 #endif