Arm backend: Add function to help linker force include backend

[zingo]

Summary

Add a function that serves as a linker force-include mechanism to
ensure the EthosU backend module gets properly linked into the final
executable, even when it might otherwise be optimized out by the
linker. This is needed when you use linker options that remove unused
code or data then this function can be called from your runner to
force the inclusion of the EthosU backend module.

This might be the case when build Executorch in other projects like
in ZephyrOS when you have an other set of compilers and linker flags.

Test plan

Tested by invoking the project from ZephyrOS and check if the backend get installed or not.

cc @freddan80 @per @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16177

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❌ 1 Cancelled Job, 1 Unrelated Failure

As of commit 8ff4a4e with merge base 670bc11 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• periodic / test-models-linux (buck2, mv3, portable, linux.2xlarge, 90) / linux-job (gh)

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
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