Add empty .s file to allow `//go:linkname`
Add empty .s file to allow `//go:linkname`

This was previously broken in 212917409, resulting in "missing function body"
compilation errors.

PiperOrigin-RevId: 213323695
Change-Id: I32a95b76a1c73fd731f223062ec022318b979bd4

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor