[lworld] RFR: 8281880: AsmTools driver uses wrong implementation classes

[Dan Smith]

In Valhalla's copy of the AsmTools library, jtreg runs the JtregDriver with a class loader that knows about the original AsmTools bundled with jtreg. As a result, these original classes are used instead of Valhalla's modified implementation. 

Fix: run the driver code in the context of a class loader that will prefer the Valhalla implementation.

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Commit messages:
 - 8281880: AsmTools driver uses wrong implementation classes
 - Merge branch 'lworld' into asmtools
 - Merge branch 'lworld' into asmtools
 - 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/650/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=650&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281880
  Stats: 141 lines in 1 file changed: 80 ins; 20 del; 41 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/650.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/650/head:pull/650

PR: https://git.openjdk.java.net/valhalla/pull/650