Build: Make Armclang 6.15~6.17 version check as an error

Commit a135228 converted Armclang version check against [SDCOMP-59788]
to an warning as users might rely on 6.15~6.17 for new features.

As Armclang 6.18 is released and fixes [SDCOMP-59788], users shall use
Armclang 6.18 instead.
Make the check as a fatal error again as the warning can be ignored or
missed easily.

Change-Id: Ib934164b1fbcfb25344c33df34aa94e558e5f9bd
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/toolchain_ARMCLANG.cmake b/toolchain_ARMCLANG.cmake
index e408622..6d2d53c 100644
--- a/toolchain_ARMCLANG.cmake
+++ b/toolchain_ARMCLANG.cmake
@@ -190,10 +190,10 @@
 
     if (CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 6.15 AND
         CMAKE_C_COMPILER_VERSION VERSION_LESS 6.18)
-        message(WARNING "Armclang 6.15~6.17 may cause MemManage fault."
-                        " This defect has been fixed since Armclang 6.18."
-                        " See [SDCOMP-59788] in Armclang 6.18 release note for details."
-                        " Please use other Armclang versions instead.")
+        message(FATAL_ERROR "Armclang 6.15~6.17 may cause MemManage fault."
+                            " This defect has been fixed since Armclang 6.18."
+                            " See [SDCOMP-59788] in Armclang 6.18 release note for details."
+                            " Please use other Armclang versions instead.")
     endif()
 
     # Cmake's armclang support will set either mcpu or march, but march gives