RFR: 7903206: Support MSYS2 for building jtreg on Windows

[Christian Stein]

Reviewed-by: ihse, jjg

(cherry picked from commit d60381de853f9e17d358b6a5c745be595f71e5c9)

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Commit messages:
 - 7903206: Support MSYS2 for building jtreg on Windows

Changes: https://git.openjdk.org/jtreg/pull/88/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=88&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903206
  Stats: 16 lines in 3 files changed: 10 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jtreg/pull/88.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/88/head:pull/88

PR: https://git.openjdk.org/jtreg/pull/88