Clean up NativeCompilation.gmk and its newly created parts

This is a follow-up to JDK-8325877. I was careful in that bug not to change order of any lines, only split up the original file into parts.

In this PR, I use the new structure to improve the design. I collect the functionality into three clearly separate phases, preparation, compilation and linking. I use consistent naming to reveal whether a function just sets up variables, or create output. I simplify and split up a few extra hard to read functions. I move some code around so it is placed more logically.