Make go:linkname work for other architectures

[antong]

Add an empty assembly file to make the go:linkname directive
work on other architectures than those that have .s files
included.

Without this I get:

netstack\sleep\sleep_unsafe.go:88:6: missing function body
netstack\sleep\sleep_unsafe.go:91:6: missing function body

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[antong]

I signed it!

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Thanks for the patch. Unfortunately, our infrastructure doesn't support accepting pull requests here. Please use Gerrit to send a CL to the gVisor repository.

[antong]

OK, I had misunderstood that this was upstream for gVisor and not the other way around. Anyway, I now noticed that the issue this PR fixes actually was already fixed for arm64, by commit_arm64.s. The naming of that file was a bit unfortunate, as naming it e.g., dummy.s would have fixed it for all architectures and not only arm64.