[lworld] RFR: 8281618: Add asmtools sources as a test library

[Dan Smith]

This is mostly a direct copy of the asmtools repo. The one interesting file is JtregDriver, which avoids various complications involved in directly invoking the asmtools 'Main' entry point.

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Commit messages:
 - Fix trailing whitespace
 - Added usage comment
 - Generalize JtregDriver for all asmtools commands
 - Add JtregDriver to provide an entry point that doesn't call System.exit
 - Add asmtools source to test/lib

Changes: https://git.openjdk.java.net/valhalla/pull/646/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=646&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281618
  Stats: 27569 lines in 101 files changed: 27569 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/646.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/646/head:pull/646

PR: https://git.openjdk.java.net/valhalla/pull/646