Issue Description:

Derived from SwiftNIO SSL issue #251.

Some Apple platforms require app submission using bitcode, a non-final representation of the binary artefact. Bitcode submissions have a number of constraints that don't apply to binary submission. One of these is that they must not contain any raw assembly code, either as blocks or as complete files. Importantly, the compiler will reject the mere presence of any .s/.S files in the build tree, even if those files are empty and contribute no object data to the build.

SwiftPM supports mixed C/assembly targets, and SwiftNIO SSL uses this support. This mostly works great, unless someone wants to ship a watchOS app using SwiftNIO SSL (a supported platform). Here the mixed C/assembly files cause a fatal build error.

We need a way to exclude assembly files using SwiftPM depending on the deployment target that does not rely on kicking those files out to a separate target.