RFR(S): 8192927: os::dir_is_empty is incorrect on Windows

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8192927

This change is targeted for JDK 11.
Please refer to the comment in the bug for a description of the change. 
It also handles path longer than MAX_PATH.

webrev: http://cr.openjdk.java.net/~ccheung/8192927/webrev.00/

Testing:
     hs-tier1, hs-tier2 on linux-x64, windows-x64, macosx
     appcds tests on the above platforms + sparcv9.

thanks,
Calvin