arm: Make gcc 6.0 or later a hard requirement now.

Move the warning to an error as we have been promising would happen in
this release.
Signed-off-by: NTom Rini <trini@konsulko.com>

arch/arm/config.mk

@@ -68,8 +68,8 @@ endif
 checkgcc6:
 	@if test "$(call cc-name)" = "gcc" -a \
 			"$(call cc-version)" -lt "0600"; then \
-		echo -n '*** Your GCC is older than 6.0 and will not be '; \
-		echo 'supported starting in v2018.01.'; \
+		echo '*** Your GCC is older than 6.0 and is not supported'; \
+		false; \
 	fi