closure: port use of dlsym to Windows
closure: port use of dlsym to Windows

This adjusts the initialization path to ensure that the lookup for
`_Block_destructInstance` is portable to Windows.

e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,alblue/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,alblue/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,alblue/swift-corelibs-foundation,alblue/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,alblue/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,JGiola/swift-corelibs-foundation