Add the ability to the make/Main.gmk build targets to produce a pure "debug" symbol image package.

Currently the JDK image does not contain debug symbols, except on Windows. To aid developers it would be useful to produce a "debug" image along side the matching jdk image. We would thus have:
1. A clean JDK image as exactly as it will go into the tar.gz by the bundles target.
2. An additional "debug" image that contains ONLY the debug symbols, for the matching jdk image.

The aim of this enhancement would be to add a "debug-image" target to make/Main.gmk to produce a debug symbol only image.