[BOLT] Add AArch64 worker and builder

[paschalis-mpeis]

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[paschalis-mpeis]

Hey folks,

This PR is part of introducing an AArch64 BOLT buildbot.

I have a couple of questions to @gkistanova:

(1) The worker name bolt-worker-aarch64 would recycled after PR #459 is merged. Or should we use a different name?

Specifically, could reusing Meta's former AArch64 buildbot name cause issues in the staging/master buildbot database?

(2) In the docs, the Steps To Add Builder To LLVM Buildbot section advices buildbot version 2.8.4, while Testing a Builder Config Locally version 3.11.7. Is the latter specific to local config testing, or should we pump the version on the guide?

Thanks in advance,
Paschalis