Force SAMD21 and SAMR21 targets to ARMC5 due to incompatibility.

The hal code for this target uses "const volatile" types inside of
structs, which are non-trivially copyable in clang (used by ARMC6). This
causes the build to fail.

Here's the commit that changed this in clang:
llvm-mirror/clang@a3d727b

It seems this was reverteed some time ago in clang, but ARMC6 may not
be up to date.

targets/targets.json

@@ -6646,7 +6646,7 @@
             "TC_ASYNC=true"
         ],
         "extra_labels": ["Atmel", "SAM_CortexM0P", "SAMR21"],
-        "supported_toolchains": ["GCC_ARM", "ARM", "uARM"],
+        "supported_toolchains": ["GCC_ARM", "ARMC5", "uARM"],
         "device_has": [
             "ANALOGIN",
             "I2C",
@@ -6679,7 +6679,7 @@
             "TC_ASYNC=true"
         ],
         "extra_labels": ["Atmel", "SAM_CortexM0P", "SAMD21"],
-        "supported_toolchains": ["GCC_ARM", "ARM", "uARM"],
+        "supported_toolchains": ["GCC_ARM", "ARMC5", "uARM"],
         "device_has": [
             "ANALOGIN",
             "ANALOGOUT",
@@ -6713,7 +6713,7 @@
             "TC_ASYNC=true"
         ],
         "extra_labels": ["Atmel", "SAM_CortexM0P", "SAMD21"],
-        "supported_toolchains": ["GCC_ARM", "ARM", "uARM"],
+        "supported_toolchains": ["GCC_ARM", "ARMC5", "uARM"],
         "device_has": [
             "ANALOGIN",
             "ANALOGOUT",
@@ -6747,7 +6747,7 @@
             "TC_ASYNC=true"
         ],
         "extra_labels": ["Atmel", "SAM_CortexM0P", "SAML21"],
-        "supported_toolchains": ["GCC_ARM", "ARM", "uARM"],
+        "supported_toolchains": ["GCC_ARM", "ARMC5", "uARM"],
         "device_has": [
             "ANALOGIN",
             "ANALOGOUT",