Adds client thread support to AArchXX. Implements dynamorio_clone for both ARM and AArch64 and adds necessary TLS swapping, including handling initial tpidr* values pointing at read-only memory.
Ports the client.signal test's assembly to a64 and arm. Enables the client.signal test on aarchxx. The test covers the client thread support here, as well as general client signal handling and manipulating. Tested manually on arm with the #4522 (closed) fix.
Issue: #4474, #4516 (closed), #4522 (closed) Fixes #4516 (closed)