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
2 files changed