[zorg] Add Arm-managed workers for AArch64 Windows builders (#936)

We are migrating the AArch64 Windows Clang and LLDB builders from
Linaro-hosted infrastructure to Arm-hosted infrastructure.

Add the new Arm-managed workers to the existing AArch64 Windows Clang,
LLDB and Clang 2-stage 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.