[zorg] Add Arm-managed workers for AArch64 Flang, LLDB and GlobalISel (#916)

We are migrating the AArch64 Flang, LLDB and GlobalISel builders from
Linaro-hosted infrastructure to Arm-hosted infrastructure.

Add the new Arm-managed workers to the existing AArch64 Flang, LLDB and
GlobalISel builders. Reusing the existing builders avoids introducing
duplicate builders during the transition and keeps the builder names
unchanged.

The new workers will initially connect to the silent Buildbot master for
validation. Once the existing Linaro workers have been taken offline,
the Arm workers can be connected to the normal master.

The Linaro worker configuration can be removed separately, once the
migration is complete.