[jdk17] RFR: 8259848: Interim javadoc build does not support platform links

[Erik Joelsson]

This patch adds copying of element-list files generated for jdk.javadoc to the interim langtools version of jdk.javadoc. I'm also refactoring the jdk.javadoc/Gendata.gmk file a bit to better adhere to current build infra standards as this was missed in the original review of this file. Rebuilding should now work better with the various clean targets and any problems with the generation tool will be captured in separate log files, along with the command lines.

@hns Can you verify that this solves the problem you reported?

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Commit messages:
 - JDK-8259848

Changes: https://git.openjdk.java.net/jdk17/pull/227/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=227&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259848
  Stats: 45 lines in 1 file changed: 20 ins; 3 del; 22 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/227.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/227/head:pull/227

PR: https://git.openjdk.java.net/jdk17/pull/227