Build race between modulegraphs and exploded-image-optimize targets

When I reorganized top level docs build targets in JDK-8206311, a race was introduced. The docs-*-modulegraph targets use the BUILD_JDK to run a build tool. For this to work, the BUILD_JDK needs to be ready for usage. In a normal native build, this means that the exploded-image-optimize target needs to be done. This dependency is however missing, so we sometimes get errors like this:

* For target support_docs_JDK_API-gengraphs__gengraphs_JDK_API_exec:
Error occurred during initialization of boot layer
java.lang.module.FindException: Error reading module: /w/jdk/build/jdk/modules/jdk.security.jgss
Caused by: java.lang.module.InvalidModuleDescriptorException: Truncated module-info.class

To fix this, I propose adding a new top level target "runnable-buildjdk". This new target will do different things depending which kind of BUILD_JDK the current configuration is set to use. The default BUILD_JDK is just the exploded image. When cross compiling, we either create a BUILD_JDK explicitly, or we use an externally provided BUILD_JDK. By introducing this virtual target, we don't need to sprinkle these conditionals in quite so many places.