RFR: CODETOOLS-7903393: Compile jtreg with -g

[Jonathan Gibbons]

Please review a simple makefile change to compile jtreg with `-g`

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Commit messages:
 - CODETOOLS-7903393: Compile jtreg with -g

Changes: https://git.openjdk.org/jtreg/pull/140/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=140&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903393
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/140.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/140/head:pull/140

PR: https://git.openjdk.org/jtreg/pull/140