[libllbuild] Add linker flags in the modulemap file
[libllbuild] Add linker flags in the modulemap file

This will allow autolinking some required system libraries when building
with SwiftPM. We should be able to move this to the manifest file once
SwiftPM gains support for declaring such things.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild