[foreign+linkToNative] RFR: Fill out missing Windows upcall stub generation

[Sundararajan Athijegannathan]

This is in response to 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-February/004199.html

Change in directUpcallHandler_x86.cpp uses "constexpr".

     https://openjdk.java.net/jeps/347 (a candidate JEP)

says

" With JDK 11, the code has been updated to support building with newer 
versions of the C++ standard, although it is not yet using any new 
features. This includes being able to build with recent versions of 
various compilers that support C++11/14 language features."

FWIW I searched for "constexpr" in current hotspot code and I didn't 
find any usage.

-Sundar