UseJava: Fix classpath separator on WIN32 cross compilation (#15206)

The condition to use ";" as classpath separator includes a check for the
switch "WIN32" which is a target switch.  When cross-compiling for a
non-windows target, this switch is not present and the separator
defaults to ":".  It should check for "CMAKE_HOST_WIN32" instead.
Merge topic 'UseJava-classpath-cross-compile' into next

7a373d5e UseJava: Fix classpath separator on WIN32 cross compilation (#15206)