RFR: JDK-8212091 : Move native code under platform specific folders and files

[Alexander Matveev]

Please review the jpackage fix for bug [1] at [2].

This is a fix for the JDK-8200758-branch branch of the open sandbox 
repository (jpackage).

- Moved native code under platform specific folder.
- Removed most usage on #ifdefs for WINDOWS, LINUX, MAC and POSIX.
- MAC define is still used in JavaVirtualMachine.cpp and Package.cpp for 
Mac specific code to filter out some arguments. I decided to keep it as 
is for now, since Mac specific code is small.
- Defines are used in Platform.cpp to initialize platform specific classes.
- Removed all pragma warning and fixed all compilation warnings.
- Removed unused code.

[1] https://bugs.openjdk.java.net/browse/JDK-8212091

[2] http://cr.openjdk.java.net/~almatvee/8212091/webrev.00/

Thanks,
Alexander