fix placement of CHECK_LIBS test for kevent64
fix placement of CHECK_LIBS test for kevent64

Need to run this check independent of the check for sys/event64
so that HAVE_KEVENT64 will be checked and defined on all platforms.

frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch