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

[Erik Joelsson]

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

/Erik