opal-ci: Additionally build with clang on Ubuntu 18.04
opal-ci: Additionally build with clang on Ubuntu 18.04

Now that skiboot supports building with clang we can use a modern distro
to cross compile using that compiler. Ubuntu 18.04 ships with clang 6,
so start with that.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

shenki/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,shenki/skiboot,qemu/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,qemu/skiboot,qemu/skiboot,stewart-ibm/skiboot,stewart-ibm/skiboot,open-power/skiboot,open-power/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,stewart-ibm/skiboot,open-power/skiboot,qemu/skiboot,open-power/skiboot