Win32: fix Java8 loading of classes

Indeed, on Windows, the separator for Xbootclasspath/p format is
";" and not ":"
4 jobs from master in 2 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #266138
debian amd64
build-debian

00:00:49

passed #266139
macos
build-macos

00:01:11

passed #266141
debian amd64 win32
build-win32

00:01:52

passed #266140
debian amd64 win64
build-win64

00:02:01