The seccomp setup code was a huge chunk of conditionally compiled
platform-specific code. For this reason, it is appropriate to move it to the
platform-specific implementation file. Ideally its setup could be moved a bit
to make it happen at the same place as the Darwin restrictions, but that change
is going to be less mechanical.
Change-Id: I496aa3c4fabf34656aba1e32b0089044ab5b99f8