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/tf_config/juno-default-armclang b/tf_config/juno-default-armclang
index bb229ea..61f1622 100644
--- a/tf_config/juno-default-armclang
+++ b/tf_config/juno-default-armclang
@@ -1,3 +1,3 @@
-CC=/arm/warehouse/Distributions/FA/ARMCompiler/6.8/25/standalone-linux-x86_64-rel/bin/armclang
+CC=/arm/warehouse/Distributions/FA/ARMCompiler/6.18/19/standalone-linux-x86_64-rel/bin/armclang
 CROSS_COMPILE=aarch64-none-elf-
 PLAT=juno