RFR: JDK-8215445: Enable building for Windows in WSL

[Erik Joelsson]

Hello,

Thanks to huge help from Andrew Luo, we now have a patch that adds 
support for building OpenJDK for Windows using WSL as the Unix layer 
instead of Cygwin. I have made some adjustments, mostly to keep it 
working in Cygwin. I have also run it through some testing, mostly to 
make sure there are no regressions to the current Cygwin support. This 
includes a COMPARE run which showed no regressions as well as comparing 
a WSL and Cygwin build, which showed no unexpected differences.

I'm happy to see that on my Windows Workstation, the time to run "make 
bundles" is considerably faster in WSL, about 8m40s compared to 12m55s.

It should be noted that testing is still not fully supported. We will 
likely need to adjust some tests to work correctly in WSL and it's also 
possible that jtreg will also need adjustments.

(For internal Oracle developers, Jib does not support WSL yet)

Bug: https://bugs.openjdk.java.net/browse/JDK-8215445

Webrev: http://cr.openjdk.java.net/~erikj/8215445/webrev.02/index.html

/Erik