RFR: 8348998: Split out PreInit.gmk from Init.gmk [v3]

[Magnus Ihse Bursie]

> The Init.gmk (and, correspondingly, InitSupport.gmk) is really two files in one: First it is called without $(SPEC), and does the work needed to locate a SPEC (and sanity check arguments), and then it calls itself again with a SPEC value.
> 
> This is two completely different tasks, and there is no reason it should be put in the same file. It just makes the file longer and harder to read and understand. 
> 
> This work is done as a preparation for fixing https://bugs.openjdk.org/browse/JDK-8292944. The code was just to hard to follow to be able to fix it without going insane.
> 
> Also, GitHub is spectacularly bad at showing the diff for this patch! I think it might be easier to review if you do it commit by commit, or possibly using the webrev frame view.

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  Even more missing include paths

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23355/files
  - new: https://git.openjdk.org/jdk/pull/23355/files/5501cf33..38925c23

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23355&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23355&range=01-02

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23355.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23355/head:pull/23355

PR: https://git.openjdk.org/jdk/pull/23355