[Executorch][llama] Split main in lib + binary

[kimishpatel]

Stack from ghstack (oldest at bottom):

• -> [Executorch][llama] Split main in lib + binary #15919

Main reason to do so is to allow internal benchmarking binary to link against main_lib

Differential Revision: D87547090

[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 947e62e with merge base aff5086 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest / linux / linux-job (gh) (similar failure)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module

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[lucylq]

nit: "main_lib" + aten_suffix

[kimishpatel]

makes sense

[lucylq]

same here, ":main_lib" + aten_suffix,