build: Make ARMClang maximum version check a warning

The maximum allowed version of ARMClang is capped at 6.14 due to a
potential MemManage fault observed with newer versions of the toolchain.
But before the issue becomes well-understood with a fix or workaround
available, we should generate a warning instead of a fatal error and
give users a choice to proceed with a newer version of ARMClang if
they need to (e.g. in order to support some newer hardware platforms).

Change-Id: I3bfcb6b7ae229886d7492fc22b8c40a2e31d54a5
Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>
diff --git a/toolchain_ARMCLANG.cmake b/toolchain_ARMCLANG.cmake
index f21fb0b..1c6d62d 100644
--- a/toolchain_ARMCLANG.cmake
+++ b/toolchain_ARMCLANG.cmake
@@ -189,7 +189,7 @@
     endif()
 
     if (CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 6.15)
-        message(FATAL_ERROR "Armclang starting from v6.15 may cause MemManage fault."
+        message(WARNING "Armclang starting from v6.15 may cause MemManage fault."
                             " The root cause is still under analysis by Armclang."
                             " Please use lower Armclang versions instead.")
     endif()