[code-reflection] Integrated: cmake tidyup and access with maven build

[Gary Frost]

cmake from hat root now delegates to backend/CMakeLists.txt, which delegates to the appropriate baclend/*/CMakeLists.txt

This has two advantages one. 
 1) The hat root cmake is way less complicated. 
 2) The backends/CMakeLists.txt is clion compatible. 
 
 I also worked out how to build cmake from inside maven.  Working for OpenCL at least. 
 
 This means that the root level cmake may well become unnecessary.

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Commit messages:
 - cmake tidyup and access with maven build

Changes: https://git.openjdk.org/babylon/pull/115/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=115&range=00
  Stats: 680 lines in 12 files changed: 298 ins; 330 del; 52 mod
  Patch: https://git.openjdk.org/babylon/pull/115.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/115/head:pull/115

PR: https://git.openjdk.org/babylon/pull/115