RFR(XS) Fixed calling convention of shenandoah_wb() C entry on Windows

[Zhengyu Gu]

On Windows with VS, the first argument is passed in RCX instead of RDI 
(Linux/gcc). There is the VS 2015 document: 
https://msdn.microsoft.com/en-us/library/9z1stfyw.aspx


Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/windows_c_call/webrev.00/


Thanks,

-Zhengyu