Convert JPRT_ARCHIVE_BUNDLE to unix style paths

When running a build in JPRT, there are couple of variable defined by JPRT that get picked up by make/Jprt.gmk, which instructs the build where to put certain artifacts specific to JPRT. Traditionally, JPRT provides these on the windows mixed mode format. This causes the build to fail unless a gnu make which understands colon is used. We are trying to upgrade the gnu make version used in jdk9 and so we would like to remove the requirement on being colon compatible.

The fix is pretty simple. Just add a check and convert the variable values before trying to use them in make/Jprt.gmk.