RFR: JDK-8170284: Move fine granular hotspot make targets to top level

[Tim Bell]

Erik:

> After the hotspot makefile rewrite and the elimination of the hotspot
> dist output directory, it's now time to create more fine granular top
> level make targets for building hotspot. This will enable better
> granularity in between hotspot build steps and the rest of the build.
> Building native libraries in the JDK will only need to wait for one
> libjvm to be built. Building docs will only require the gensrc step in
> hotspot to be built.
>
> Here is the full list of new targets proposed:
> hotspot-gensrc
> hotspot-libs
> hotspot-<variant>
> hotspot-<variant>-gensrc
> hotspot-<variant>-libs
> hotspot-jsig
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8170284
>
> Webrev: http://cr.openjdk.java.net/~erikj/8170284/webrev.01


Looks good.

/Tim