Build: Upgrade ArmClang version to 6.21

Previously, ArmClang was upgraded to 6.18, but this version was found to
have issues when UBL licensing is enabled. So, upgrade to the latest as
of now to resolve them.

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

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I2f6dffbdd8079cc22e0016c9be247aed508eb02a
diff --git a/performance.py b/performance.py
index 97c6554..e377898 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_18_PATH')
+        os.environ['PATH'] += os.pathsep + os.getenv('ARMCLANG_6_21_PATH')
         if os.getenv('CONFIG_NAME') in mem_configs.keys():
             print('Configuration ' + os.getenv('CONFIG_NAME') + ' is a reference')