[code-reflection] Integrated: Hat add jextract to maven

[Gary Frost]

Previously we modified the bld to use jextract, here we updated the mkpoms script to generate maven equiv code. 

So 

. ./env.bash 
java @bldr/mkpoms
mvn -e clean compile jar:jar install

Should create backends and examples and extract opencl and opengl for nbody example.

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Commit messages:
 - swap hat.target for hat.build in pom.xmls and cleanup mkpoms
 - mkpoms now generates jextract capable pom.xml files
 - pommaker now works with extractions

Changes: https://git.openjdk.org/babylon/pull/291/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=291&range=00
  Stats: 486 lines in 27 files changed: 329 ins; 14 del; 143 mod
  Patch: https://git.openjdk.org/babylon/pull/291.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/291/head:pull/291

PR: https://git.openjdk.org/babylon/pull/291