Warn with ARMC6 and not v8m

tools/toolchains/arm.py

@@ -25,6 +25,7 @@ from tempfile import mkstemp
 from shutil import rmtree
 from distutils.version import LooseVersion
 
+from tools.targets import CORE_ARCH
 from tools.toolchains import mbedToolchain, TOOLCHAIN_PATHS
 from tools.hooks import hook_tool
 from tools.utils import mkdir, NotSupportedException, run_cmd
@@ -368,6 +369,14 @@ class ARMC6(ARM_STD):
         if target.core not in self.SUPPORTED_CORES:
             raise NotSupportedException(
                 "this compiler does not support the core %s" % target.core)
+        if CORE_ARCH[target.core] < 8:
+            self.notify.cc_info({
+                'severity': "Error", 'file': "", 'line': "", 'col': "",
+                'message': "ARMC6 does not support ARM architecture v{}"
+                " targets".format(CORE_ARCH[target.core]),
+                'text': '', 'target_name': self.target.name,
+                'toolchain_name': self.name
+            })
 
         if not set(("ARM", "ARMC6")).intersection(set(target.supported_toolchains)):
             raise NotSupportedException("ARM/ARMC6 compiler support is required for ARMC6 build")