build(make): update arm/clang to latest versions

Update our testing framework to utilise Arm Compiler v6.18. On jenkins
and local runs, the LLVM toolchain is downloaded and its path prepended
to the PATH variable so that the host machine can find the latest tool
along with with its binutils.

Change-Id: Ie1c984ba01e3a8e0666e1e1f35b9303293b032cb
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/group/tf-l1-build-clang/fvp-default-clang:nil b/group/tf-l1-build-clang/fvp-default-clang:nil
index aab4796..0428f17 100644
--- a/group/tf-l1-build-clang/fvp-default-clang:nil
+++ b/group/tf-l1-build-clang/fvp-default-clang:nil
@@ -1,6 +1,10 @@
+#!/usr/bin/env bash
 #
-# Copyright (c) 2019-2020 Arm Limited. All rights reserved.
+# Copyright (c) 2019-2022 Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
 
+pre_tf_build() {
+    setup_llvm_toolchain
+}