[code-reflection] Integrated: add extern C and minize square output

[Gary Frost]

Mock backend was broken, compiled, but fgailed to link at runtime. 

I added `extern"C"` to getBackend method.

Also cleaned up/minimized output from squares and violajones examples.

Ensured that `java @hat/clean` removed cached src dirs (this needs refactoring).

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Commit messages:
 - ws
 - add extern C and minize square output

Changes: https://git.openjdk.org/babylon/pull/434/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=434&range=00
  Stats: 17 lines in 4 files changed: 9 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/babylon/pull/434.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/434/head:pull/434

PR: https://git.openjdk.org/babylon/pull/434