Fix CMakeLists.txt for Windows

[cmcgee1024]

This change prepares swift-crypto for use on Windows with CMake and also the Package.swift for use in the Swift toolchain CI system.

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

If you've made changes to gyb files

• I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

Upgrading swift-crypto from an older version requires that Windows works properly with CMake, and also SwiftPM.

Modifications:

Modify the CMakeLists.txt files and the generator script so that it conditionally adds assembly files for non-Windows platforms. Prepare the Package.swift file for use in Swift CI pipelines, by consulting an environment to either use the absolute URL, or a prepared version of the swift-asn1 dependency.

Result:

After this change it should be possible to build with the newest swift-crypto in the Swift CI system.