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/script/build_package.sh b/script/build_package.sh
index 80b58b6..d13d24b 100755
--- a/script/build_package.sh
+++ b/script/build_package.sh
@@ -502,6 +502,13 @@
 		set +a
 	fi
 
+	if is_arm_jenkins_env || upon "$local_ci"; then
+		path_list=(
+			"$llvm_dir/bin"
+		)
+		extend_path "PATH" "path_list"
+	fi
+
 	cd "$tf_root"
 
 	# Always distclean when running on Jenkins. Skip distclean when running