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.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

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