RFR(S): 8185694: Replace SystemDictionaryShared::_java_platform_loader with SystemDictionary::is_platform_class_loader()

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8185694

The open code change for this fix involves adding the 
_java_platform_loader to the SystemDictionary class.

webrev: http://cr.openjdk.java.net/~ccheung/8185694/webrev.00/

Testing:
     JPRT
     hs-tier3 on linux-x64, macosx, solaris-sparcv9, windows-x64.

thanks,
Calvin