reorganize VOLK_IMPLEMENTATION usage

[SamGondelman]

Summary: when using ETVK with volk, Runtime.cpp should not define VOLK_IMPLEMENTATION, but should instead leave its definition up to clients. this makes combining multiple volk-dependent libraries easier

Differential Revision: D87851728

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 9d42fe0 with merge base 4014597 ():

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

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

• pull / android / run-emulator (gh) (trunk failure)
  Timeout waiting for emulator to boot.

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[meta-codesync]

@SamGondelman has exported this pull request. If you are a Meta employee, you can view the originating Diff in D87851728.

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