Build: Upgrade ARMClang version to 6.18

To support UBL licensing technology, CI needs to upgrade ARMClang
version to 6.18. Old version compilers are going to be deprecated.

https://linaro.atlassian.net/browse/TFC-525

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I6ea966fcf1ab18c7c4df7a5271720a164855801f
diff --git a/performance.py b/performance.py
index cae2e7e..3fa96b4 100755
--- a/performance.py
+++ b/performance.py
@@ -144,7 +144,7 @@
 def main(user_args):
     if user_args.generate_memory:
         # Export ARMClang v6.13 to ENV PATH
-        os.environ['PATH'] += os.pathsep + os.getenv('ARMCLANG_6_13_PATH')
+        os.environ['PATH'] += os.pathsep + os.getenv('ARMCLANG_6_18_PATH')
         if os.getenv('CONFIG_NAME') in mem_configs.keys():
             print('Configuration ' + os.getenv('CONFIG_NAME') + ' is a reference')