Enable customizations in CompileJavaModules.gmk and Main.gmk

Changes made to the pre-hook and post-hook in CompileJavaModules.gmk (as part of 8258407), as well as the include directive for Java.gmk using an absolute path, leaves no path to customizations.

The proposed fix is to change these files (in the jdk stream) to use either the include path (a-la 8252998) or PHASE_MAKEDIRS, as appropriate.

The proposed fix also includes a suggestion for a minor tidy-up, while we're patsubst-ing things.

https://github.com/openjdk/jdk/pull/2134