[code-reflection] Integrated: Cleanup of HAT env.bash, bld and hatrun scripts

[Gary Frost]

Added a jextract check in env.bash 

hatrun's usage now shows way to specify backend (i.e. ffi-opencl vs just opencl) in preparation for jextracted backends

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Commit messages:
 - Cleanup of HAT env.bash, bld and hatrun scripts

Changes: https://git.openjdk.org/babylon/pull/309/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=309&range=00
  Stats: 24 lines in 3 files changed: 11 ins; 1 del; 12 mod
  Patch: https://git.openjdk.org/babylon/pull/309.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/309/head:pull/309

PR: https://git.openjdk.org/babylon/pull/309