[code-reflection] Integrated: implemented preprocessor hook for OpenCL code gen

[Gary Frost]

Here we leverage OpenCL's ability to lean on the C99 preprocessor to generate C99 code. 

Instead of having OpenCL generator differentiate, we get the OpenCL code builder to define a set of #defines and generate backend 'neutral' C99 code (common between Cuda and OpenCL). 

This PR just has the OpenCL changes.  Cuda changes will be next

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Commit messages:
 - implemented preprocessor hook for OpenCL code gen

Changes: https://git.openjdk.org/babylon/pull/600/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=600&range=00
  Stats: 250 lines in 10 files changed: 90 ins; 44 del; 116 mod
  Patch: https://git.openjdk.org/babylon/pull/600.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/600/head:pull/600

PR: https://git.openjdk.org/babylon/pull/600