RFR: JDK-8244945: Mark VS2019 as supported and default

[Erik Joelsson]

When updating the Windows toolchain at Oracle, I forgot to also mark the 
new toolchain VS2019 as supported in configure. This has the effect that 
configuring with VS2019 causes a warning to be printed.

We should also make configure prefer 2019 over 2017 if both are available.
Bug: https://bugs.openjdk.java.net/browse/JDK-8244945

Webrev: http://cr.openjdk.java.net/~erikj/8244945/webrev.01/index.html

/Erik