bionic-amd64-tf-m-build: Install ArmClang 6.21

Primary motivation at this time is to test is there's an improvement
re: discovered issues with ArmClang 6.18 UBL (produces binaries which
fail LAVA tests).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I52fe9d03b18b45d63ed09ebc66bf3431e38ccd62
3 files changed