[code-reflection] RFR: Deprecated APIs for accessing GPU global thread information removed.

[Juan Fumero]

This PR refactors the `KernelContext` to remove the old accessor `kernelContext.x` and `kernelContext.maxX` to a `gix` for thread builtins and `gsx` (as in global size or x) for specifying the total number of threads.

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Commit messages:
 - minor change
 - [hat] Native CUDA thread dispatch fixed
 - [hat] OpenCL native dispatch refined
 - [hat][wip] towards thread-id unification

Changes: https://git.openjdk.org/babylon/pull/656/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=656&range=00
  Stats: 290 lines in 38 files changed: 26 ins; 65 del; 199 mod
  Patch: https://git.openjdk.org/babylon/pull/656.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/656/head:pull/656

PR: https://git.openjdk.org/babylon/pull/656