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
2 files changed