Migrate backends/arm to the new namespace (#5904)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5904

Move the Arm backend out of the `torch::` namespace, and update to avoid using the `torch::` or `exec_aten::` namespaces.

Also, move the VelaBinStream code into a namespace to avoid declaring symbols in the global namespace.

Reviewed By: mergennachin

Differential Revision: D63923290

fbshipit-source-id: a98e4c1ede8072a9fd96fb7fc2e2fcc4e82bc28a